I have two kids and my oldest 4 and half has great interest in my Paintball addiction. Would I let him play, nope. For a few reasons.
1- Minimum age allowed by Insurance company regulations is 10
2- If he did get hit for some reason the marker FPS fluctuated and it hurt him hey may be turned off to paintball all together.
3- Trying to convince other parents to let their very young kids play could prove to be impossible. I have a hard enough time convincing adults it is fun.
4- Markers and equipment are not really made for someone so small, goggles mostly. I know some of you said you have ones that fit, but fit well and fit are different. If my son lost an eye I would never forgive myself
5- I don't care how mature a child acts, they are still a child and they may not 100% adhere to the rules. Many of you are young enough to attest to that.
6- Unlike your case I have a wife that would kill me if I took my son out to play now, or even before he turns 10. (don't get me wrong, when he is close I will let him shoot a few hoppers at a poor defenseless tree, when she is not home
So should she play? Well in the big picture that is up to you, but those are my opinions and I am sticking to them