WHEN ITS TIME FOR YOUR CHILDS FIRST HAIRCUT, CALL THE YELLOW BALLOON. On any given Saturday, this festive childrens hair styling salon is a hive of activity. There are 13 stylists on the floor, sniping and shaping hair and making children feel like theyre at a birthday party. Free balloons, cookies and candy, a popcorn machine, a large play area with a mini-arcade and toy boxes at each salon chair take away from the trauma of a haircut. "Our stylists have had years of experience with children before coming here, " says Assistant Manager Christina Kirilova. "They entertain the kids with stories, toys and even magic tricks, so that they forget why they are here." Annette Bening and Warren Beatty have brought their youngsters here, as have Varma White, Pete Rose and Eddie Van Halen. The Olson twins and child stars from "Big Daddy" and "Seventh Heaven" are among the salons pint-sized patrons. For a babys first haircut, The Yellow Balloon includes a framed Polaroid picture commemorating the occasion, a certificate and a lock of the babys hair in a special envelope. Yellow Balloon 12448 Ventura Blvd., Studio City ; 760-7141, -KT.