My gut level feeling is that you are asking, "will I be caught if I cheat like this?" The answer is "no, it's unlikely". Kids are not usually queried as to their age.
We go to DL frequently, and have never seen anyone stopped. In fact, we went with friends who brought their 3 1/2 year old twins and did not buy tickets.
Having said that, I would probably do it (buy a ticket, I mean). Our last trip was in 2001, when my youngest ds was 3 1/2. He was (and is) small for his age, and we could have easily passed him off as under 3. I just felt funny lying about it.
It's really about what you will and won't do.
On the other hand, I have a 5 + 4 hopper plus pass I bought for my oldest ds last year during one of Ticketmanias specials. Problem is, ds is over 9 and was actually over 9 when I bought it. Truthfully, when I ordered over the phone, I wasn't even thinking about ages, I just ordered 2 adults and 3 kids (he was ten at the time......a kid to me!)
I'm not sure what to do now. He shouldn't use that ticket, but I hate to buy another pass.
So, I have my own dilemma.
Julia