Where will all the children go after school?

Starting this year to leave my 10-year-old home alone in the morning and to have him come home alone in the afternoon has been very unnerving. This ''nice'' neighborhood ... presents many situations that require more insight and careful thought than I think should be required of a 10-year-old. ... A center to go to or activities to attend would be a great service.

That's how one parent answered a recent questionnaire about the need for before- and after-school child care in her community.

To listen to the concerns of more than 250 mothers and fathers who also responded to the New Bedford, Mass., survey is to be convinced of the mounting challenges facing so-called ''latchkey children'' and their families.

As youngsters across the nation head back to classrooms this week, an estimated 6 to 10 million of those between the ages of 5 and 13 will be fending for themselves once the school day has ended. The figures aren't new, but the growing awareness of what they mean is new.

In fact, Sept. 2-8 has been designated National School-Age Child Care Awareness Week, the first ever proclaimed by presidential edict. Another first: the ''School Facilities Child Care Act,'' a ground-breaking federal initiative to provide money for school-age child care - $15 million worth - has been passed by the House of Representatives and was recently reported out of committee in the Senate, according to an aide to Sen. Donald W. Riegle Jr. (D) of Michigan, sponsor of the bill.

''Five years ago school-age child care was like a black hole in outer space - it was not an issue that lots of people identified with,'' says Dale Fink, an associate of the School-Age Child Care Project at the Wellesley College Center for Research on Women. ''Now it's become a new criterion for families on the move. It's not just who has the best school system anymore, but who has appropriate care for children after school. Families are moving out of towns that don't have it, and into towns that do.''

The only program that deals with school-age child care on a national basis, the Wellesley Project gets inquiries daily from all parts of the country. Researchers hear from state legislators who are trying to document the numbers of youngsters needing care, and they answer calls from school administrators who want to know how their facilities can be used to best advantage.

Although schools have perhaps the greatest potential as sites for after-school use, the range of program models is wide. Programs may be housed in day-care centers, churches, and social, community, and recreation facilities. Some, like the Living Center in Kansas City, Mo., combine all of the above.

To mark this national-awareness week, the Living Center is opening 10 new programs for youngsters. At several YWCA sites, children can make ice cream, conduct flea markets, and take part in talent shows for aerobics and break-dancing. Swimming lessons, gymnastics, karate, computer play, ceramics, and field trips round out the activities offered by participating community groups.

''There's a tendency to think that children are best off when one parent is available at home,'' says Dale Fink. ''But from my own experience as a provider of child care, I think that for many children it's more stimulating and fun to be in a setting where they're exposed to a lot of different things that no one parent is capable of organizing.

''The thing about parents is that they're there to love you and give you affection and give you a chance to talk their ears off, but a good after-school program provides that, too.''

According to a policy report published by the Wellesley Project, one of the most interesting developments in the delivery of school-age child care is the initiative increasingly being taken by individual parents.

Just ask Barbara Bivens. A Massachusetts mother who has chosen to be at home with her daughter, now a fourth grader, she says she spent one too many afternoons during the last school year as resident mom-on-the-block for youngsters whose parents worked.

Over the summer, Mrs. Bivens and two former teachers got together to do something to respond to what she calls the ''total neglect'' of young children left at home to care for themselves.

After sending out a questionnaire to parents of elementary school children in their hometown of New Bedford, they found that 20 percent of the 1,200 families they contacted were in need of help.

Since then, the three women have talked with state social services providers and written their congressmen about the possibilities of federal funding. They soon will present their plans to the local school committee, with hopes of opening their doors next month.

''I personally feel that there's nothing better than a mother being at home with her child,'' Mrs. Bivens notes. ''But I also know what kind of person I am, and what kinds of interests my two friends have to offer my daughter, and I think that together we would be much better for her than I am on my own.''