Betsy's good idea--to earn money by running a summer play group for preschoolers--turns limp if not quite null in the execution. Put six tots together, mix in a few assorted pets--including an imaginary friend's imaginary dog--and some funny mixups are bound to occur. A real Thumpy who piddles is more trouble, for instance, than a make-believe Tinkie who, insists friendless Rodney, doesn't (""You can't say what Tinkie does. I say. Nobody says but me""). Also in the cards is a disaster and a constructive response. But the garden burial of Sammy's beloved chicken Mona--run over by a car--is so mechanical as to demean the occasion (the weak pictures at least show some feeling). More dubious still is the dispatch of Rodney's ""Bobo"" and ""Tinkle"" that brings the book to a thudding close. Through contorted circumstances, Rodney wins a real dog, and then he acquires a real, loudmouth friend (to wit: ""Yippie, I'm glad I'm real""). A Bobo by another, less clownish name would have been harder to dislodge, we'll warrant. But small children really do have special tastes in peanut butter and jelly, kittens do get into cute scrapes, and--a passing thought--these books still give you a run for your money.