Catnapped!.

The story involves a boy and his younger sister whose dog Papadoll is missing. They find out that he has been abducted by anthropomorphic cats to be brought to a world called Banapal Witt, which is built on top of a giant sleeping cat. There the sun turns poor Papadoll into a bizarre giant monster. He becomes the weapon and beloved pet of a spoiled Princess who turns everything she touches into balloons. She plans to use Papadoll and a giant inflatable mouse to control the sleeping cat and thus Banapal Witt itself.