THE BERENSTAIN BEARS AND TOO MUCH BIRTHDAY and THE BERENSTAIN BEARS: NO GIRLS ALLOWED, both by Stan and Jan Berenstain (Random House: $1.95; 32 pp. each; ages 3-6).

What greater opportunity for parents to go overboard than at their child’s birthday? Nothing’s too good for Junior, but it might be too much as we learn from those funny Berenstain Bears of the pink-trimmed tree house who’ve entertained preschoolers for years with their mishaps.

Now Mama and Papa Bear outdo themselves when their daughter turns 6 and they mastermind a bash, her very first party. And bash it is with ponies, a merry-go-round, gifts, games (Spin the Bottle for 6-year-olds ?), prizes, a crowd of noisy friends, candy, cake, you name it. But as it often is with “first times,” something backfires, and here it is the birthday girl who ends up sobbing. Too much of a good thing is a moral perhaps intended for parents, though young readers are reminded of the importance “to learn, to have fun, to grow in every way.”

In these new books, the Berenstains stray from the rhyming verse familiar from their “The Bike Lesson” and “The Bear Scouts,” using small-print narrative instead, with their typically cheerful drawings. In “No Girls Allowed,” Sister wears a pink bow in her hair, but the stereotype ends there when she outruns, outclimbs, and wins all the marbles off her brother and his buddies. Her gloating victory dances prompt the guys to build a secret clubhouse on an island in Frog Pond complete with a drawbridge, watchtowers and--can’t blame ‘em--a “NO GIRLS ALLOWED” sign.

What follows is minor warfare, sibling style, and another moral from the ever-wise Mama: “The important thing is not whether you are a boy or a girl, but the sort of person you are.” The preaching slides by in good-natured humor and even though these little bears are irritatingly named the impersonal “Brother” and “Sister,” these 8x8 paperbacks are among the best buys around.