Q)My little girl, who is 2 1/2, is very lucky to have three neighborhood friends her age. She plays with them all, but has become very fond of Sarah, referring to her as "my sister" or "good friend Sarah."

Unfortunately, Sarah's family will be moving across the country in three to four weeks.

How do I prepare my daughter for this big loss? How soon do we start talking about the move? Would a going-away party help?

And what about moving day? Should we be around to wave goodbye as they drive away?

A)Let your little girl be part of the farewell, starting now. After all, Sarah is her best friend.

Altogether it will be a positive experience if you treat it as you would any other expected change: in a positive, matter-of-fact way. The emphasis should be placed on the adventure that's ahead for Sarah, not the friendship she'll leave behind.

Your daughter can't, of course, understand such an abstract concept as a cross-country move -- or any move at all. She will, however, get a sense of the impending separation if you talk about the move in the most simple terms, and if you keep your anxiety in check, so she can do the same.

You can help the four children translate the abstract idea into a concrete one by helping them fill a suitcase with dress-ups. First Sarah packs for her big trip, and then each of the children pack the bag, as they go off to visit Sarah.

With your encouragement, they'll deliver mail to each other if they have the paper and old junk mail envelopes. They'll also call each other, using their blocks as telephones.

A farewell party for Sarah is another good idea, if it's quite understated, or a small mother-daughter party for Sarah and her mom, with presents to make the long trip a little easier.

Chalk and an 8-by-10-inch blackboard are good and so is a hand puppet; a paperweight that will snow inside when she shakes it and anything that will talk to her without driving her parents batty.

Audiocassettes make good gifts if the family has a portable player or a tape deck in the car. Look for the classic children's stories recorded by Caedmon ($8.98) and especially the splendid Raffi tapes, like "Singable Songs" (A&M Records; $9.50).

Books, of course, are always welcome. Sarah should like one of the small, rigid picture books by Fiona Pragoff (Doubleday; $4.95) and if her mom can read while she rides, there's The Gingerbread Boy (Knopf; $4.95) and Froggie Went A-Courting, retold by Chris Conover (Farrar, Straus and Giroux; $12.95). Mr. Little's Noisy Train by Richard Fowler (Grosset and Dunlap; $10.95) -- a lift-the-flap number -- also is good, if you reinforce the flaps with tape before you give it, and The Family Car Songbook (Running Press; $4.95).

If possible, use your camera to remember these last few weeks. Take a roll or two of film, order a double set of prints and glue each set of pictures firmly in a sturdy album, one book for your daughter and one for Sarah.

Sarah's move also will be much easier for your daughter if you help her find another good friend soon. If you don't, you're going to have some difficult playtimes ahead and may think it's because Sarah has left. That would be only part of the picture. An even number of children always play together better than an uneven number.

And when the big moving day does arrive, take your child to Sarah's house to wave goodbye, like the good friends that you are. It may make your daughter a little sad, but it will make Sarah feel better. It will also gently close the relationship, which is always necessary, even when you're 2 1/2.

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