In my household we watch a lot of MMA as well as a fair amount of individual martial arts events. I myself am a very new BJJ white belt and I LOVE IT. As someone who is uber-competitive it has become a great challenge to learn something new and compete in a combat sport with out the relative chance of injury that some of the other martial arts are subjective to (I attend a great school run by Rodrigo Munduruca and everyone that I have rolled with has been very competitive and respectful).
In fact it has carried over into the "wrestling" that I do with my daughter, who watches a lot of events with me and especially loves to watch the women fight. She is more interested in the bjj and wrestling aspect and often asks me what just happened and asks me to teach her. While I readily admit I am no sort of trainer I think I can handle showing a 4 year old the basics.
I say all that to ask "at what age is it safe to have kids participate in bjj?"
Munduruca has a kids class that starts at 5 and I think I will get my daughter into it sooner than later.