All the more reason to teach them.
And all the more reason to make it MMA, not some traditional self-defence style IMO. 4 & 5 year olds should be learning wrestling skills, takdowns, pins, escapes, etc. And maybe some basic boxing. They most definately should not be learning groin kicks, eye gouges, throat strikes, kicking the knees, etc.
They also should definatetly not be learning about fighting through TV, where people land nothing but what would be crippling blows and no one ever even gets bruised (in G - PG rated fights) or worse, video games or pro-wrestling.
And if they are wrestling every other day in class, taught how to do it, what's safe, whats not and all that. I'd imagine that if it ever was "real" they'd be a lot better equipped to deal with it without getting hurt, as well as hurting the other person. Plus, my experience is that once people learn to fight, and fight in class, they tend to be less likely to get into a "real" one.
Kids will be exposed to fighting in some form or another, Don't you think it would be better that they are exposed in a controlled, safe, and fun environment. Taught how to fight properly, and safely rather then believing it is like tv, movies, WWE and video games?