How about you let in 2 or 3 every weekend on a trial basis where parents must be present at all times (maybe even so far as to they must be playing on the field with you at all times) and that you let them know it's a privilege to be playing not a right. You won't hesitate to kick people off the field and "ban" them (including 18+ players) for an indefinite period of time (ie. 1 month ban, 3 month, 1 year, etc.).
Don't be all hardcore pseudo-military on them cause that just makes things look bad (ie. "oh that guy just plays airsoft cause he can't be in the real military"), but stay strict on safety rules and douchebaggery.
In short, unless you want to do it then don't because a lot of commitment is required and it's usually more trouble than it's worth in general especially now since "shooting sports" are involved (ie. a similar thing probably would have gone on for paintball 10-15 years ago)
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE