Using copyrighted characters in decorations at a day care center is a fucking horrible idea. You’re essentially pushing marketing on those children and their families for free, regardless of how popular or ubiquitous those characters actually are. The one I worked at had rules against it and we still had countless Disney and Dreamworks books on the shelves in all the preschool classrooms, nobody really thinks about the implications beyond “kids love this stuff!”
I understand that most children really like Frozen. That doesn’t mean you should put Frozen stuff in your classrooms, because as obviously innocent and well-meaning as your intent is, it sets a goddamn horrible precedent.
(Related story: I worked with a teacher who was very nice and very overworked once. She was great with the kids but she herself got so caught up in this mentality that, on one shelf in the room, she had a poster showing a bunch of easily recognizable red objects to teach color names. These objects included a fire truck, an apple, and a fucking branded Coca-Cola can with logo facing out.)