I have shot with younger kids. I even lead a bunch of them for the last LZ game. I don't have a problem with the kids, on a personal level, most of the time: The problem comes with the legal association with shooting at a minor.
If I took that kids eye out because his goggles had failed, I would feel horrible, knowing he was not of the age of consent to play (regardless of parent's permission).
Secondly, I don't have two settings while playing, I don't have "kid friendly mode". There are some things you shouldn't say / do around kids which is 100% fine as an adult (insert chuckles, but seriously).
I think these are the two main sources of troubles with shooting with kids, other than maturity issues, that a lot of people face.
Ár skal rísa, sá er annars vill
fé eða fjör hafa. Sjaldan liggjandi úlfur
lær um getur né sofandi maður sigur.