Proof that there are still new stories to be found in the natural ways of young children, if--like newcomer and teacher Barbara Greenberg--you know them well enough. The Bravest Babysitter turns out to be not hockey-playing, happy-natured, competent Heather but Lisa, her charge, who takes the initiative when Heather's fear of thunder threatens to bring their shared activities to a halt. ""My mother always tells me to keep busy. . ,"" remarks Lisa; and with no further ado, she suggests one after another distraction. Praising her bravery, Heather calls her ""a very good babysitter,"" which allows Lisa to wish for thunder ""the next time you come so I can take care of you again."" Diane Paterson's pictures--in Anita Lobel pastels--are less-than-usually overbearing and very distinctly attuned to the quietly wise, inherently warm text.