We recently went though this with a fundraiser my Rotary Club was running. It was an Oktoberfest, and we charge admission (because the live bands cost money and otherwise you get people who sit there for 5 hours watching the entertainment without buying any food or drink and then leave). We wanted to do Adults $5 each, and children free. We went back and forth about how to determine a childs age and what constitutes a child. The end result was we said "the heck with it" and let the parents decide. If two 40 year olds want to come with their 18 year old kid and call him a child, then so be it - keep your $5. Its a charity fundraiser event, and if people want to try and get you for $5, that's on them. Of course, if you wanted to drink, you had to be proofed, and then you were clearly not a child. The end result was, many families ponied up the $5 for their kids with no problem, but we had a few teenagers who came in as "kids"
I realize a WDW ticket is way more than $5 and WDW is hardly a chairty fundraiser, but the point is, some people are great about it, other try to get every last penny out of you. There really is not much you can do about it.
-dave