SUDDENLY it is the children's hour on Capitol Hill, and high time, too.

The focus has been unrelentingly on senior citizens, a clamorous, high-turnout constituency. But now attention is being given to the small people at the other end of the scale. Statistics show that while it has been "morning in America" for some, many children are being brought up in a kind of twilight where the hopes and chances for life, liberty and the pursuit of happiness are minimal.

They go to school unarmed without the home education in numbers and letters that the more fortunate have. According to Richard Riley, former governor of South Carolina, who brought about a dramatic turnaround in his state's lamentable education system, 40 percent of all low-income children without any pre-school failed first grade. More often than not, they drag through elementary school, always behind luckier children. They are in many cases the teen-age parents, school drop-outs, jobless and imprisoned of tomorrow.

Since 1978, the number of children living in poor familes increased by 2.5 million. Some 500,000 of them are homeless. Noteven half of the homeless children go to school.

Finally, Congress has decided that it's neither good nor smart to close our eyes. Just as the country is gradually coming out of the comfortable assumption that the homeless are for the most part mental patients, unwisely turned out of institutions, sensible people are saying that we can't just let children be punished all their lives for the crime of being born poor.

Last Wednesday, Sen. Edward M. Kennedy (D-Mass.) summoned four governors and ex-governor Riley to testify in behalf of his bill for an ambitious pre-school program that would provide year-round, day-long care and education for the deprived pre-kindergarten set, an enlargement and intensification of the popular and effective Head Start program, which just barely survived the Reagan onslaught

The governors (Cuomo of New York, Kean of New Jersey, Blanchard of Michigan and Perpich of Minnesota) were unanimous about one thing: Spend or pay later.

A pilot program in Michigan -- the Perry Pre-school Project -- shows that their graduates were a third less likely to be dropouts , half as likely to have had a teen-age pregrancy half as likely to be on welfare and much less likely to be in trouble with the law.

On Thursday, Rep. Dale E. Kildee (R-Mich.) held hearings on a child-care bill that he and Sen. Christopher Dodd (D-Conn.) are sponsoring. It would provide $2.5 billion in federal funds. It is an acknowledgment that not all non-working welfare mothers are lazy -- that many simply need to have someone to look after their children while they are on the job.

The same day, on another floor of the Rayburn building, Rep. George E. Miller (D-Cal.), chairman of the Select Committe on Children, Youth and Families, presided over a packed, emotional session on the rights of homeless children, who, if they go to school at all are two years behind grade level.

"We no longer have the excuse of not knowing," said the star witness, author Jonathan Kozol, who is one of those most responsible for ending the ignorance. He is the author of an excruciating series on the homeless of New York, which first appeared in The New Yorker and has been published as a book called "Rachel and her Children."

The Republicans on Miller's committee seemed a bit defensive about the testimony. Rep. Dan Coats of Indiana noted that unprecedented amounts were being spent to help and that he was hearing "an indictment of the people running the programs." But Miller said angrily that even if the money were "spent better" only 20 per cent of the eligible children would receive compensatory education.

Kozol volunteered that he had spoken with many congressmen from both parties about the need for federal intervention and had been asked, "Can you really solve these problems by throwing money at them?" To which he replied, "'Yes, that's the American way."

At the Kennedy hearings, Riley was confronted by Sen. Strom Thurmond (R-S.C.), who made the familiar arguments against a new federal program and planted the flag of states' rights. "Federal control follows federal money," he said sternly. But Riley replied quietly that South Carolina could not afford to take the steps necessary to rescue the sinking children.

Thurmond spoke of the folly of adding to the federal deficit of a trillion dollars. How could Riley advocate spending more money?

"You can't blame the 4-year-olds of South Carolina for the federal deficit," replied Riley, and Thurmond had nothing more to say.

As to why Congress has suddenly come to about the plight of the children, Kozol presented this theory:

"After eight years of institutionalized meanness, a bipartisan revulsion has set in." Mary McGrory is a Washington Post columnist.