Temporarily banned from getting cR? (Halo: Reach)?

Okay, here's the situation.

I came over to babysit my sister's boyfriend's kids, whom are 11 and 9. I spent the night and my older brother brought over the Xbox. We decided to play Halo: Reach for a little while. I tried to teach them how to play, but the only one getting kills was James Robert, the 11-year-old. Emmy, the 9-year-old, wasn't having as much luck as the rest of us, only getting 0 kills every game. After about three hours of game play, every one of our accounts were "temporarily banned from earning credits". We weren't trying to cheat or boost, we were just trying to have fun, it was just bad luck that Emmy didn't happen to get any kills. Was this because the lack of kills? Or was it some other thing that we happened to do wrong? Can you help us to fix this?