they can lose the trademark if they don’t try to defend it

This is true, but that's if another company is using a similar logo as their own. Like, if a pet store used the Mickey Mouse logo, of course they're going to be sued.

If a daycare uses Mickey Mouse to decorate their classroom, Disney doesn't have to sue because the trademark isn't be used separate from Disney. The Daycare, and kids, are using it because it's Disney, so there is no confusion about trademark ownership.

At the very least, Disney could simply write them a letter allowing them to use depictions of Disney characters inside the school so long as it's not for advertising or commercial purposes and the art is done by a student or teacher.