My youngest that will race will actually be 6 in March as well. I'm working with him regularly, and he shows promise. He can deal with small rocks and any climb that the PW is capable of. If his attitude is good that day he should do well. My older son will be 9 in March too. He has a KLX110 but would rather play the common sports with his friends. It kills me that his best friend lives in a PERFECT location for riding but his Mom (one of my wife's friends) is a PIA that hates bikes and has the kid brainwashed. Dad is a, well, has no say.
My wife is cool, no problem with the kids riding or racing, as she rides herself occasionally ('02 GG 250 Pampera). I used to be a big time shipwreck diver, and thats the one thing she won't encourage if the kids ask about it.