Q. My husband and I have one son who will be 4 next month, and it's beginning to look like we won't have another child.

I have been undergoing infertility treatment for 2 1/2 years and we're running out of money and options.

We have also looked into adoption, but the agencies we've contacted either say we're ineligible because we already have a child, or they quote fees that range from $15,000 to $25,000. This just isn't affordable after the infertility treatment.

It's hard to talk about the infertility -- and it's private -- so I give vague or noncommittal answers to people who ask about our plans to have other children, and only talk about the infertility to family and very close friends.

But what do I tell our son? He loves babies and often says he wants a baby brother or sister. He seems too young to really understand, but I think he deserves some kind of answer.

We love our son very much and know we are lucky to have him, but still our family feels incomplete and I worry about him being lonely. And how do I protect him from the stereotypes that surround only children -- that they're spoiled, and that their parents don't like children very much?

A. Forget about stereotypes and the people who believe in them. You can't take these folks seriously. You want another child because you love children, and that's what counts.

Forget the stereotype about spoiled or lonely onlys, too. If you teach your son to give as well as receive, he won't be spoiled, and if you rear him to fit your style -- and his style -- he'll be content. A child's happiness depends on his parents, not his siblings.

You also have to forget about the wishes of your son. He won't be old enough to decide on the size of his family until he's old enough to have his own family.

At this point he's not even old enough to understand how babies are made -- no matter how clearly you explain it -- because a 4-year-old can't think in abstract terms. As one study reported, even 8-year-olds thought a baby -- or baby parts -- came from a baby store, although their parents had told them all about sex.

Now, when your son begs for another baby, you say, "It would be fun to have a baby around, wouldn't it?" or "Only if the baby would be as sweet as you," or, quite simply, "Why?"

You'll probably find out that his wish is just a romantic notion, typical of the convivial 4. He may be enchanted with the new baby down the block -- or with the presents the new baby's brother has gotten -- or maybe he wants someone to play ball with him. This just tells you that your son wants a little more company, a little more action, a little more conversation -- and all of it is a little more than a baby could give him, even if there were a baby around.

You can make up the difference by sending your son to pre-K five mornings a week and by letting him invite a buddy for an overnight once a week or on some family jaunts.

It's really your aching heart that needs assuaging, not his. You'll find many of your concerns covered well in "Parenting an Only Child" by Susan Newman (Doubleday, $9.95), but if you still can't reconcile yourself to the situation, you may have to expand your family in other ways.

Adoption is the traditional answer, but the odds with an agency aren't great. Only 60,000 couples are chosen every year, out of 2 million applicants.

A private adoption, generally handled by a lawyer, is much like an agency adoption. The court still requires home studies, references, and proofs of marriage and financial stability, but an independent adoption is usually much quicker and requires less red tape -- or perhaps the lawyer just makes it seem easier to cut.

Private adoption is also more expensive, as you've found, although the fees you cite are definitely on the high end.

According to "The Private Adoption Handbook" by Stanley B. Michelman and Meg Schneider with Antonia van der Meer (Dell, $8.95), it costs between $3,000 and $20,000 to adopt independently, depending on the cost of the lawyer, medical expenses and travel, and whether the mother needs financial support during pregnancy. All of these costs are approved by the court and paid by the adoptive parents, even if they -- or the birth mother -- cancel the adoption at the last minute. It's a pitfall to consider.

There are other ways to have a child. Many handicapped or abandoned babies and older children are institutionalized for want of a home.

If you took care of a foster child in your home -- even on a short-term basis -- you and your husband would feel more fulfilled, and so would your son. A 4-year-old may not understand much about sex, but he does understand about people, and he would empathize with them, if he could walk in their shoes.

It can feel as good to reach out to other children as it does to your own, and it feels mighty good to the "reachees," too.

Questions may be sent to P.O. Box 15310, Washington, D.C. 20003.