It can be done
As some of you know, I fly a lot of aerobatics. My kids have also been doing acro with me for years, and it has been a very positive experience for them. My youngest son was 8-years old when he started flying aerobatics with me, and now he's talking about flying in aerobatic contests after he gets his PPL. We have "standard operating procedures" and full-up emergency procedures training for the children. I am not casual about any of our flying, especially aerobatics. When a child is ready to be signed off as an acro pax, then we go fly.
My humble opinion is that many here are over-thinking the subject. If your passenger a) fits properly in the parachute harness, b) is fully trained on how to use it, and c) can egress the airplane, then go fly. Get a smaller parachute for lightweight individuals with a crossover aerobatic harness. Tighten the straps all the way to the stops, and if it's too loose, then the passenger is too small to fly, period. This isn't rocket science folks. Feed the kid steak and potatoes until he/she fits the chute harness.
I should add d) the most important thing: first look in the mirror. Are you trained and proficient? Can you responsibly carry a passenger in aerobatic flight without causing unnecessary risk? The standard of excellence is much higher when carrying a young passenger.
Finally, there's aerobatics, and then there's passenger aerobatics. Loops should not be round, but rather cursive "L" shaped. Slow rolls should be docile and fun. G loads should be as low as possible. And the flight should be short. The idea is to have a positive experience where the passenger is still smiling in the base turn.
Karl, Goodyear, Arizona (KGYR) ATP, CFII
=VAF= donor 2021