Along with the interest in science fiction for older children, there seems to be a trend in this age group to exchange for the season the riding crop for the magic wand of fantasy, and this story of a marvelous baby sitter is enticing fare. Strange things happen when Mr. Pudgins comes to baby-sit with John (8), Janey (6) and Pete (4) -- things of which the grown-ups are only dimly aware. First there was the bear that settled on Janey's bureau and wouldn't disappear until he gave her a bear hug; then the mirror children, exactly like John, Janey and Pete except they floated, the Dodo bird who spoke a strange language and was easily offended, Mr. Pudgins' car Annabelle which swam and leaped. Then there was the day the house faucets ran soda pop and the night the bathtub flew. Straight-faced under-playing and very real children turn the trick. Pert, appropriate sketches by Margaret Bradfield make it all very convincing.