I have five year old boy/girl twins. The boy has two older brothers and is fearless to a fault, plays hard and holds his own. The girl we padded up with homemade armor and she is not afraid of getting a hit it but prefers target practice to games. She can bullseye a six inch bell from 50 yards every time. Proud daddy
I think the biggest point is to pad the heck out of the younger ones so they don't get scared and turned off at the start.
I almost forgot your original question. Go to Walmart and get a $7 dollar camping pad, the ones that go under your sleeping bag, then buy a roll of tape-on Velcro. You can put a suit of great armor together if you have some old coveralls or sweatsuit hanging around. Then let her borrow your gear and blast away!