From his somewhat undignified position on the diaper-changing table, Jason Allen, 2, told everybody in sight at the Child Care Development Center about his encounter with a caterpillar. It had just wiggled a bit in its glass container, jarring Jason's composure.

"Gad-a-pea-yer! Scare me!" he repeated, wide-eyed.

The caterpillar was headline news later that night at home, said Jason's mother, Sharon Allen. Each day at the new child-care center in Ellicott City, a long-awaited facility that opened recently for children of Howard County school employees, has brought similar excitement to Jason and his brother, Evan, 4, their mother said.

"They talk about the songs and the stories . . . the blocks . . . drawing on the blackboard . . . . There is a vast array of programs," said Allen, a speech pathologist at Harper's Choice Middle School and Clarksville Elementary School. "They're excited, and I'm excited."

In a county where nearly two-thirds of the women are estimated to be working outside their homes, demand for child care is high. A facility to serve school system employees has been a priority of the Board of Education for several years and was recommended a year ago by a school task force.

The new center, in the Howard County School of Technology on Route 108, next to the school system headquarters, has initially attracted 26 children. It is licensed to take as many as 68 children ages 2 to 4.

"We didn't have anything for people to come and look at until very recently. And people had to make arrangements further in advance," said center director Julie Ibson, a veteran day-care administrator in the county. "Twenty-six is a very comfortable starting number. We will add others gradually without any difficulty."

Parents, who include teachers, custodians, administrators, secretaries, counselors and other staff members, pay $85 a week per child, or $158 for two, rates considered average for the county.

Start-up costs for the nonprofit center are being paid by the school board. And space was made available in what was once a computer lab at the technical school.

The four large rooms, filled with bright new toys and equipment, open onto a fenced play yard at the back of the school. Prominently displayed are the names of the children who attend, such as Emma, Zachary, Caitlyn, Adam and Alex.

The initial staff of five, including several teachers with degrees in education, was selected from among 250 applicants, Ibson said. Along with a "high-quality program . . . parents were looking for consistency in staffing, because a child can become so attached to a care-giver. Staff turnover is a chronic problem in day care," she said.

While cooperative, parent-organized nursery schools have long been a component of life in the planned town of Columbia, little company-based day care for preschoolers has existed in the county.

Last year, Trusted Information Systems, a high-tech firm in Glenwood, opened a facility for young children of its employees; Howard County General Hospital plans to open one next year.

In 1988, a county government report recommended that employers expand child-care benefits, a move it said could help attract more workers.

Sharon Allen said her husband, an administrator with the University of Maryland, dropped Jason and Evan off the first day at the new center because "I am the emotional one -- I would be crying.

"Evan went right to the toys and started playing like it was Christmas day, because everything was brand new. Jason wanted to hug goodbye and get a kiss, and then he went to play too."

Allen knows this because her husband videotaped the whole thing, the camera tucked under his arm during the hug.

She said she feels "secure" about the school because its teachers are "the cream of the crop." She and the other parents have been told to feel free to stop by any time.

"I came in around noon one day, and they were finishing their lunch. I was able to tuck them both in for nap time," Allen said. "The open-door policy is something a lot of parents will feel comfortable with. They encourage it."