[asid61]

I was thinking something like heavy cardstock would work, although they would have to reprint them fairly often. That would make it less cost if kids lose them, however, especially if you have to outsource printing on plastic.
Photo paper might last a bit longer, although I'm not sure on that.
Some companies do plastic card printing, online. Starting with that and using cardstock replacements in cases of losing them could be a good idea.