I'm having trouble believing it - can I be the only person on Goodreads who's come across this excellent children's book? Lulu, about five, wants to play in the snow... but her baby brother is cold, so Dad takes them off to the art gallery. Lulu's in a foul mood, and makes herself so unpleasant that Dad says she can damn well stay on her own until she's calmed down.
Suddenly, two winged cherubs turn up, escaped from one of the paintings! They grab hold of Lulu and pull her back into the art world with them, Mary Poppins style. Posy, an accomplished artist in her own right, has a lot of fun doing kids' versions of well-known pictures and showing you Lulu's reactions. At the end, she decides that the gallery isn't nearly as dull as she'd first thought.
My own children loved it when they were small - Posy's touch is light, and she's more concerned with keeping the young reader entertained than with preaching about Great Painters. It's enjoyable to read aloud too. Strongly recommended!