I think that Animorphs are considered childrend book 'cause the general idea is about aliens, superpowers, science fiction, animals, and the protagonists are teens.
The most childish thing IMHO is the representetion of the Yeerks as a race of brutal, ruthless, evil beings, which seem to hate one another - I don't understand how they can cooperate for an invasion, in these conditions - and Visser III, that is a very stereotyped villain, he seems to act as if he were playing the role of the Bad Guy, bad just to be bad.