Goggles on, or get out.
I am a player in my early twenties and I go to the field to play, and I'm sick of fretting over getting lawsuits every time some moron walks around a game with his face exposed. My pet peeve with goggles go with everybody, not just kids. I see people older than me with their masks off inside a field all the time.
For your children's safety, make sure they wear good masks ($50 range seems to be adequet, JT and Proto are good brands in that range) and the field is strict about their velocity limit. Often, the refs are just kids. Watch out for incompetent refs and go to the field with the least of them.
Gun is overrated. $200 and under is good for beginners, and the kid should try a rental/borrow from a friend the first time to see if he/she likes the sport enough to invest. Don't fall for the impulsive "can I have that $500 Shocker? it shoots better!" crap. If the kid is still acting that immature, nothing good can come from him having that gun.
Understand that a gun is an equipment and needs routine maintenance.
A paintball can take out a kid's eye as easily as a grown-ups.