Online interactions are NOT rated by the ESRB. The ESRB is the rating given due to the content of the game. Your petition would be completely pointless.
Secondly, you have given Rare what, 4 days? 4 days to go through thousands of different submissions of toxic players, glitches, etc? Wow. I would at least have given them a full work week before I went to the forums to say "Shame on you, Rare!"
I'm very sorry that this happened to you and your children. It is never pleasant when dealing with toxic players. However, blaming Rare in any capacity is misplaced and unfair. Rare does everything they can to punish toxic players and gives plenty of tools to keep someone from encountering such things.
There is an option with Microsoft to set an account to a "Child Account". If you are worried about your kids playing, simply give them a "Child Account" and you can limit what games they play based on age, time spent playing, as well as who can interact with them and how. I believe this would stop other players from communicating with your children, if you are concerned about that.
Another step you can take is to mute other crews. This way they won't be able to talk to you if you don't want to hear such things.
If you have not taken such steps and have not taken the time to educate yourself on how the ESRB actually works, as well as not giving a company more than 4 days to communicate back to you about something like this (also doesn't mean they haven't done anything about it, just means they haven't told you about it) then shame on you.