Can anyone explain to me the ending of the Children's book 'Oi Puppies!' by Kes Gray?

My daughter has this book and at the end of it is the cat and the dog asking what tadpoles and kittens without mittens sit on, the frog asks why before being chased by tadpoles and kittens then he calls for a taxi. Is there an obvious joke I'm missing or is the ending just a bit rubbish? Edit: I have not read any of the other books in the series so I don't know if it's related to them