An estimated 3.5 million American 5-year-olds are entering kindergarten this year, and more will be attending full-day classes than ever before.

Ten years after the first nationwide experimentation with "full-day" (6-hour) kindergartens, the still-controversial programs are spreading so fast they are now the norm for at least a third of all kindergarteners across the country, according to U.S. Census figures. The fraction may be closer to half, some suggest, given full-day's rapid growth since 1983, when the Census figures were last collected.

While the changeover is widely viewed as one of the most important recent developments in early childhood education, its impact and its purpose -- custodial or educational -- are the subjects of fierce debate.

In the Washington area, D.C. public schools, with all 284 kindergartens on a full-day schedule, and Montgomery County public schools -- with 63 full-day kindergartens -- have shown the greatest enthusiasm for the concept.

Of Montgomery County's 102 public elementary schools, 25 have full-day programs this fall, double last year's number. How fast the county can meet parent demand for more depends on schoolroom space and budgets -- both more highly taxed by the longer-day program.

Dismissing a basic objection to full-day programs, Ken Muir, director of long-range planning for Montgomery schools, says, "I think most educators and our board of education feel children not only can take more school at an early age, but that many of them really need it . . ."

The Arlington, Fairfax and Prince George's County public school systems so far offer only traditional 2 1/2-hour kindergarten classes.

Full-day advocates, including many kindergarten teachers, say the longer day allows greater flexibility and more time for enrichment activities, play, prescribed academics and individual attention. They say it has been of special help to children with learning problems or lack of English fluency.

"I would never want to go back and teach 2 1/2 hours," says Audrey Caldwell, full-day kindergarten teacher at New Hampshire Estates Elementary in Silver Spring near the Prince George's County line. "I just don't think I could do that. It really is a program that helps children."

Parents whose children have attended the programs tend to agree.

Kathleen Dewey of Rockville, for example, transferred her youngest son out of his home school district last year to enroll him at a full-day program farther away. "I purposely put him there because it was all-day. My oldest son [12] went to an all-day program and did well . . . My middle child [7] went to half-day. I feel he had a very weak start, and he's struggling. I don't know if there's a connection, but he wasn't reading when he went to first grade. The other two were . . . They also really liked it."

Critics tend to see the program more as an attempt to meet parents' needs -- especially day-care needs of working parents. "To be quite honest about it, some of it comes from public demand for custodial services for children," maintains Brian J. Porter, spokesman for Prince George's County public schools. Michael Castleberry, who coordinates an early childhood education project at George Washington University, is harsher: "I don't think [the growth of full-day kindergartens] has anything to do with education," he says.

Outspoken opponents such as Louise Bates Ames, 76, associate director of the Gesell Institute in New Haven, Conn., known for its research and clinical work in early childhood development, also question children's physical readiness for the full-day programs.

" . . . Most 5-year-olds are not ready for all-day school," says Ames. "We feel many 6-year-olds are not ready. Many 5-year-olds still need a nap in the afternoon. With all-day school, they fall asleep. We heard from one teacher who said, 'It's so hard to wake the kids up from their naps.' I said, 'Why do you have to wake them up?' She said, 'So I can teach them. That's what they pay me for, to teach.' "

Critics also contend the full-day curriculum subjects young children to too much early academic pressure. Says Ames, "We feel if kindergarten is held all day, it's bound to become more academic. Their eyes are not ready for it, their hands are not ready for it, their brains are not ready for it. They need much more gross motor development than they have. A good many children are struggling . . ."

Public kindergarten -- begun in this country in St. Louis in 1873 and still not mandated in all 50 states -- already is more academic than it was a generation ago. Educators acknowledge it underwent enormous changes in the last two decades. But the steady trickling down of math and reading skills once reserved for first grade, most say, predated the full-day movement and grew out of massive changes in family and social patterns.

The institutionalization of day care and nursery school has helped produce worldly wise preschoolers, says Jacquelyn Levine, full-day kindergarten teacher at Rock Creek Forest Elementary School in Chevy Chase, Md.

Today, says Levine, "many children come to school with readiness skills I taught 29 years ago when I first came to the county. I actually had to teach children all the things we take for granted when children come to school now. They can dress themselves. They can tell you their name, address and phone number. They know the letters of the alphabet -- we used to teach that in kindergarten. They can read numbers. That's also a product of TV, of "Sesame Street." Their world is bigger today. Twenty-nine years ago, children didn't go much beyond their house."

Full-day advocates cite the same social realities to bolster their cause.

"It used to be thought that 5-year-olds could not take an all-day program," says Rock Creek Forest principal Louise Rosenberg, "and that they really needed more time at home with a smaller group doing more appropriate things than academics.

"I think now the reality of the life style throughout the country is children are away from home in day-care arrangements or nursery school practically from birth because there are so many working parents. Even parents who are not working have their children in nursery school by the age of 2 or 2 1/2. It's the rare child who's not in some nursery program by the age of 3. Therefore it doesn't make sense for educators to say children can't take the program when they've already been before they come to school . . ."

Longer days -- in the form of extended day programs -- are becoming standard for children of working parents even in school systems without full-day kindergartens. Arlington County has them in all 18 of its elementary schools, Prince George's County in six, Fairfax County in 47, D.C. in 30. In Montgomery County, where the establishment of several such programs has been left to individual schools, administrators are investigating a systemwide extended day program.

Closely tied to this and the full-day kindergarten debate is the question, already receiving wide attention, of providing public-school programs for 4-year-olds. District public schools already have 175 half-day and 45 full-day prekindergarten classes.

One of the few longitudinal studies of students attending full-day kindergartens suggests the children indeed can take it and take it well.

The 4-year study of about 400 Evansville, Ill., public-school children, half of whom attended full-day kindergarten in 1978 and 1979, claimed this group had positive attitudes toward school, higher academic and conduct marks, a lower rate of being retained and higher standardized test achievement scores than peers who attended traditional half-day kindergartens.

Experts agree, however, that results like these hinge on an imaginative, well-coordinated program, not just on a traditional curriculum stretched into a longer day. "There's no question curriculum needs to be addressed as part of the planning procedure; it's often neglected," says Susan Cruikshank, a co-coordinator of this summer's second annual symposium, "Implementing the All-Day Kindergarten," at Columbia University Teachers College in New York City.

Some schools seem to have found a mix of academics, enrichment and play that works.

At New Hampshire Estates Elementary, where full-day kindergarten was begun in 1977, principal Barbara Frank says children are not pushed to do more than they can handle.

"I've seen all-day kindergartens where there are reading groups and children sitting with paper and pencil very early on. That's not necessarily what we do at New Hampshire Estates. We're very child-oriented . . . We do a lot with manipulative materials . . . a lot of active learning, hands-on activities. We are not paper-and-pencil oriented . . . Those who are ready read, but those who are not ready, are not asked to do it. We're not trying to put a first-grade curriculum into a kindergarten setting."

The new program, says Frank, has "required a lot of adjustment on the part of the teachers. They have to plan their year so the very beginning is very different from the end of the year in the amount of time spent in any one activity . . . Developmentally, children are not ready to sit for a long time."

If it's as good as proponents claim, what's to keep full-day kindergarten from spreading even faster? Cost, for one thing, and space for another.

In Montgomery County, where Superintendent Wilmer S. Cody says the goal is to make full-day available to all parents who prefer it, the total cost per child in a half-day program is $2,873. The cost per child in a full-day program is $3,748 -- 30 percent higher.

According to Ken Muir, the system's director of long-range planning, "We estimate we would need 135 more teachers and 270 more classes to make all-day kindergarten available to all children. That would take five years at $600,000 a year more. We're talking about a $3 million program.

"The big problem is we're going to run out of space. There are 74 schools now that do not have full-day kindergartens. Only 30 would have space for it. So, how fast we move in full-day kindergarten is going to be dictated by how much room we have in the schools, and enrollment is growing, so the space problem is going to get worse before it gets better."

The future? It depends on whom you ask.

Says Ames: "I think it's very dangerous. However, in my lifetime, I've seen a lot of nutty things take place in the schools and if they're bad enough, they fall their own way eventually."

"I'm not seeing it as a fad at all, counters Cruikshank. "I think it's here to stay . . . All-day kindergarten is almost going to be a fait accompli in four or five years. It's gaining acceptance that rapidly."