Unfortunately, you can. The problem is *selling* items made with any images which are copyrighted, etc., though whether you *would probably* get in trouble in this situation is a different issue.
Most companies won't go after (or even know about) smaller or one-time sales, but some are notorious for doing that even at craft fairs (like Disney).
The same is true for other images --like school mascots (and even school colors when used in conjunction with anything school related that's sold), images created with rubberstamps (though there are many "angel" rubberstamp companies) and images from books, etc. --there are often a limited number of those that are allowed to be sold without specific permission however.
(You can generally use those images for your own things --no selling or profitting-- but I think that even that is "technically" illegal.)
I think the kids would go for other kinds of "character-y" pumpkins though, that weren't specific characters. In other words, just add your own weird decorations for accessories, faces, etc., and don't worry about trying to duplicate anything they already know (though you could grab a general idea here or there).
HTH,
Diane B.