Q. I know an 18-month-old child is too young to have playmates, but my daughter becomes extremely upset when another toddler comes to visit. She cries, fusses, clings and is panicky when someone else starts playing with her toys. The situation is little better when we're the visitors.

I have taken her to playgroups and I'm a full-time mom who never leaves her with a sitter. So it couldn't mean she's worried about my leaving.

My husband and I also fear what her reaction to a new baby will be. What can I do?

A. Young children do have friends, but they seldom share with them well at 18 months, and they share even less well at 2 and especially at 2 1/2. However, your daughter is a bit extreme, even for her age and stage.

The question is why, and the answer may be in you.

Being a full-time mom is great, but if you've never left your child with a sitter, she's had little chance to test her mettle. She still has to learn how to work out problems by dealing with people on her own.

This doesn't mean that you should plunge her right into a social whirl. Your daughter is probably shy by nature, and since she hasn't had much experience with playmates, she needs a slow introduction to learn how to interact. Even then she may not be ready for nursery school until she's at least 3 and perhaps even 4. Children don't follow the same schedule.

It will be easier if she starts with sitters once or twice a week when you go to the movies or for a drive. But take care to choose an adult or teen-ager sensitive enough to be gentle, and lighthearted enough to keep her complaints in perspective.

You'll also want to keep your role in perspective by remembering who you are rather than what you are. A part-time paid or volunteer job could help reinforce a sense of yourself, but you'd be wise to limit your work to one full day a week, rather than several days. So much shuffling could be unsettling to your little girl. A single day away can give her the comfort of a stay-at-home mom, plus some of the independence a working mother requires of her child.

Occasional absences are true compliments. They will tell your child that you know she can take care of herself, which in turn will ease the pressure she must feel. No matter how hard parents try, an only child often feels as if a laser-like intensity is beaming down, which is sure to cause anxiety and the need to control whatever she can. In this case, her toys.

She protects them because at this age they are an extension of herself, as a car might be to a teen-ager or money to an insecure adult. She can share her toys only when she has enough confidence in herself to let them go. Or when she feels you trust her enough to let her go, which means a little pushing from the nest.

You might forget about having children over for a while. There's no point in expecting her to be ready to share better, just because other children do. You wouldn't make her give up her bottle, crib or diapers to follow the schedule of her crowd.

Instead, your child needs some encounters of another kind. Visits in more public settings with more impersonal surroundings should help most. The library, the ice-cream parlor and the playground are ideal places to arrange your visits, especially if your child leaves all her toys at home and friends are kind enough to have their children do the same.

Toys, books and equipment are easier to share when none of the children owns them, and they are used on neutral territory. This will give your child the self-confidence to develop good friendships; she'll learn how to negotiate and compromise with her peers.

And will she react to a new baby? She certainly will. No only child likes to share her parents, and with good reason. Parents, after all, are a child's favorite toys.

Questions may be sent to Parents' Almanac, Style Plus, The Washington Post. Worth Noting

Two magazines for parents:

Children Today, published by the Department of Health and Human Services, informative, thoughtful discussions six times a year for $14. Superintendent of Documents, U.S. Government Printing Office, Washington, D. C. 20201. Include stock number 017-090-80001-8. Genesis, published by ASPO/Lamaze for professional and parent advocates of natural childbirth, $15, which includes national family membership; six copies a year. ASPO, 1840 Wilson Blvd., Arlington 22201.