Firstly, gambling is illegal for minors. Secondly, have you ever seen slot machines? Players like to pick machines they feel comfortable with. This is Zynga using their own branding to diversify into another product.
The one thing to be said of online casinos is that they offer good protection against underage gamblers. Unlike a fruit machine in a pub, you can't just wander by and anonymously put money in. The payment systems generally prevent registering/depositing by under-18s. And even the Gibraltar gambling commission (of which Zynga falls under) is very strict about preventing minors from playing.
In Las Vegas, there are slot machines with all sorts of cartoons (Disney characters, etc). But if you look under 18, and you walk up and touch them, security is going to appear out of nowhere and ask you to leave.
There are PopCap themed slot machines already. Their characters are just as "kid friendly" in the sense that they are simple and stylized, but again PopCap's products are for adults and not kids.
