Plot holes
I never saw this movie as a kid, so I don't know if these points would have bothered me or not. Probably not. But as an adult viewer:
1. Why doesn't the "witch" let the children know she has Thomasina? As she saw their elaborate funeral and knows that it was a named cat, she must realize that Thomasina was very special to someone. And she had plenty of time to learn whose pet she was.
2. This one's worse IMO. How, after going to great lengths to make the adults aware of the circus staff's cruelty to animals, can the kids go to a performance and apparently have a great time watching? (There's a shot of them laughing and enjoying themselves.)
I'd guess that #1 had a reasonable explanation in the book but was glossed over in the film. #2 is just plain wrong.