I would say a big part of this book is about the food chain. the little brother gets swallowed by a creature, and the big sister embrakes on a journey to save him. and each and every time she almost makes it another creature comes and eats the previous one. but she does eventually makes it, and save...
A tasty treat with an entertaining spin on "there was an old lady who swallowed..." Great for reading aloud.
Would make a great read-aloud. Lots of nonsense names of imaginary animals.