A golden-haired moppet tells how her big brother scares away all the monsters that lurk under her bed, in the bath (wearing snorkel and flippers, ``he fights off all the sharks...and squashes them down the drain'') and in the treehouse. But when it comes to real frogs, John's a wimp and his sister has to help, tenderly hiding the frog ``where John can't see him.'' This fine new twist on an old theme hails from Australia. In Niland's cheery color-pencil illustrations, the imaginative monsters, compliant frog, and kids are equally appealing. (Picture book. 3- 7)