My son is 7 and is currently a Blue belt in TaeKwonDo. He has been in it for almost 2 yrs. I think it's great dicaplin for the children and as for the sparring of it, that's part of the training. They need to learn to block hits, where to hit in defense etc etc. They have the gear on. It's soooooo good for them.
I say go for it! I can't wait to put my daughter in it when she's old enough!