Q. "My son is 6 months old. When he was 3 months old I went back to work fulltime. Since then he has been going to a sitter's house. She cares for him, for a little girl the same age, and for her daughter, who is a year old. My son is a good-natured and happy baby and he seems to be doing well in this situation.

"Now my sitter has decided to work one day a week and has offered to find someone to take the children for that day, or says I can get someone new.

"I want to do what's best for my son and I'm concerned about the continuity of sitters and the stability of his environment. I'm even considering quitting my job although I'm not sure that is a good long-run solution for him or myself. (I earn a good salary and it provides the extras and eventually will pay for his schooling.)"

A. Stability is very important to a baby and so is love and enrichment. Since your child is so happy he must be getting all of this now. This is because he basically has two good families -- his own, which always will be special, and his sitter's. The two have given him such a firm base it shouldn't bother him to stay with someone else one day a week, particularly since he'll be with his friends.

Certainly you would be going out one day a week if you stayed home, although the time might be spread between Monday and Friday, with an hour here, a lunch there, an afternoon somewhere else.

It will be time enough to make different arrangements if this replacement sitter is a wash-out. As long as you don't have to change too often your child should do just fine.

Q. "I'm considering nursery school for my child, who will be 3 in September.

"She's very alert and vocal, talks very well, and is partially potty-trained. She gets along well with other adults and plays well with children, but she dosn't have many children to play with except the baby, who is 6 months old.

"I though that two afternoons a week with other boys and girls might be good for her, but I wonder: How necessary is nursery school?"

A. Nursery school isn't necessary, but it is a big plus.

A child needs to know other children, so she can interact with them and learn the social skills she will need in kindergarten. It's hard for a child to get this at home, where she's bound to be something of a princess.

Your daughter also will need more challenges by the time she is 3 -- more than you can give with a toddler underfoot. As one reader recently wrote, "Nursery school is the time for free play, which does more to develop school readiness than any learning games."

A child learns by doing, and in a good nursery school there is always a lot to do. Even though you give your child many projects and field trips, a nursery school is sure to have more, for it draws from many sources. It's the novelty -- and the contrasts -- that give a zest to living.

You'll need to register now (it's already too late for many schools) and it may be hard to find an afternoon program, and for good reason. Most children like to play hard in the morning and rest or nap in the afternoon.

Your child will make many new friends in a nursery school, and you probably will too, for there is much involvement of parents, especially in a cooperative where mothers and fathers take turns helping the teacher.

Two or three mornings a week also will give you time alone with your baby, and he will love it. Every child needs a little time to be an only child.