Pennsylvania Jones said:

I'm not the biggest fan of the prince calling him an ass EVERY SINGLE TIME HE REFERS TO HIM!!!

I know! The Prince is such as jerk, how dare he call the animal what it is...

I have a funny feeling that if Puzzle was a duck, and the prince constantly referred to him as "duck" every single time, it wouldn't bother you in the slightest, would it?

In other words, I assume your problem doesn't stem from the prince not taking the time to learn Puzzle's name, or his lack of creativity apparent by failing to call him a wider variety of names, but from the fact that in American English "ass" also happens to be a dirty word and you are uncomfortable with reading it in a children's book.