The underlying problem is simple. Minors present a significant legal risk for the owners and occupiers of the property, the organizers of the game, and other players. If a minor gets seriously injured at an airsoft event, someone is likely to have to pay big.
Waivers do not reduce this risk for having a minor participate, no matter who signed it. Having the parent on site with the child reduces the risk, but does not eliminate it.
Is there some compelling reason I should risk my house, car and investments so an underage player can participate in this sport?