If your child is smaller or higher wieght, they still have a great chance at wrestling a 9 year old.
Have your 6 year old try the 8&u bracket as a B and wrestle the A bracket at your next tournament and find out in a hurry if the he or she is ready.
Many 6 year olds have went up to 8&u for subs and some have placed and even won state before.
As long as they are safe there is really nothing to lose. If the 6 year old wins, they beat the big kids, :p yea!! If they don't succeed then so what the they got beat by the big kids, nice try.