Oh, yes. In fact, I heard a story about a full-grown man who tried to get into the park with a child's ticket, and actually tried to argue with the CM that he was nine years old ("No, I don't have any ID. I'm nine."). And no, it didn't work.
The worst that will likely happen is that the CM at the gate will deny her entry, and you'll have to purchase a ticket for her while getting disapproving glares from the other guests who actually obey the park's rules and policies.
Here's a post from another message board I read occasionally:
Last year Universal got a talking Donkey who does a meet and greet next to Shrek and Fiona. He is a talking puppet, who is very funny and usually draws a small crowd. Sometimes we Shrek attendants can hear Donkey through the wall, and will poke our heads out to see what Donkey is saying. Today we saw this:
Donkey: So how old are you princess?
Girl: I'm three
[Donkey notices that the girl's mom is holding up TWO fingers, not three]
Donkey: I see your mom over there is saying two. So what is it? Two or three?
Girl: Well, I'm three, but Mommy said that I was two to the man.
Donkey: Mama lied to get you into the park for free!!! Mama, you owe me money! You owe me money!