I am not familiar with the series, but I can give you some guidelines to evaluate it:
1. Do the characters engage in fantasy magic (i.e., magic that cannot be replicated in real life, such as riding a broomstick), or do they engage in occult magick (i.e., magick that might be conceivably attempted in real life, such as divination or necromancy)? If it is occult magick, then the series would be inappropriate for Catholic children (cf.
CCC 2116). If it is fantasy magic, similar to
The Wizard of Oz for example, then it may be okay.
2. Even if it is fantasy magic, there may be other problems with the books that bother your husband. (For example, while the Harry Potter series by J. K. Rowling generally contains only fantasy magic, some adults worry about the young characters’ disregard for rules and the truth.) Ask your husband to be more specific about what bothers him in the Animorph series so that you can review the books together and come to a decision that is right for your children.
Recommended reading:
Harry Potter vs. Gandalf by Steven D. Greydanus