When I was a kid, years ago, I think the only mistakes I made getting into paintball was the lack of knowledge about the guns. If you are going to get a gun for your kid, make sure it is:
Dependable, meaning you wont spend an arm and a leg paying the local shop to fix it all summer. Tippmann model 98 is my vote, I've had one for 6-8 years and replaced the $12 seal kit once, nothing else done to fix it, and it works well everytime.
Resaleable, meaning that if your 12 year old decides that next month he/she wants to be a hockey player or whatever, the gun could get you some money in resale. How to avoid this? Don't buy a platic gun, at least not in my opinion. Metal always sells better. If you are worried about this, make them rent a few times first to see that they will stick with it.
Safe, meaning that most entry level markers also come in a kit for less than $100 more that includes a mask and a barrel condom/sleve.
Also, train your kid to keep their mask on when the referee says to, and the barrel condom/sleeve on when the ref says to. Biggest and most annoying mistake that young kids make at my local fields is taking their mask off when they aren't supposed to. Repeat this to them "keep your mask on, listen to the refs" as much as you would say "put on sunscreen" when they go to the beach. Dying of skin cancer is a ton less likely in my opinion than getting shot in the eye with a ball at 250 - 285 feet per second. It hurts and can kill, but if you LISTEN to the refs and experienced players, it NEVER will happen.