It is only truly a legal problem if you intend on selling the finished item.
If you are concerned about copyright, please check out the crafty business advice board. There is always lively discussion regarding that topic.
Keep in mind, though, that it is a THEME. The project is supposed to be based on the theme. It could be a necklace inspired by a TV character, a sculpture of what you think a book character looks like, or even a piece of wall art. It just has to be primarily made of polymer, air dry, or metal clay.
I worked in collections for years, and one of the stores we collected for was The Disney Store. We opted to hand-draw an "award" for top collectors. NOTHING was sold, and it was hand drawn by an employee so nothing was technically reproduced (as in putting a Disney coloring book on a copy machine....)
When the "big wigs" from the company came and saw those certificates on desks, they made everyone who had one take it down and throw it away. Like, INSTANTLY.
This is one red flag I have going off.
I'm not trying to harp, or negate anyone's tallents on these threads. I think people do AMAZING work on here. It would seem that receiving a prize is the same as getting paid. Again, not trying to be a thorn in anyone's side, or a nay-sayer. I'm just concerned.
I may have an "answer"-ish type thing for this.
as in collections, do you mean collecting money? monetary involvement is a red flag definitely. I had created a beaded wall-hanging from a video game sprite (you may or may not have seen it as I did display it on here quite a while ago) and I was interested in entering it in a Fire Mountain contest. I e-mailed them asking if I would need to get permission to enter it or if it was ok to just enter it. they told me that since there was the possibility of monetary gain, I would need the permission of Disney AND Square Enix (since Disney owned "The Little Mermaid" and Square Enix created the sprite for their video game). if I didn't get permission, and someone from those companies saw it or someone reported me, I could get in trouble. so in the end, I didn't enter it (too much hassle, never knew if I'd get a response in time, plus I doubted myself and figured I wouldn't win anyway). not so sure about the "inspired by" part though...
in the end, it's all up to the mods. I do hope this helped in some way.