Just ask Dave!!
He put his 9 year old son on an xr100.
He re mounted the rear shock on a different mounting hole, and cut two inches off the forks.
Its really low to the ground.
Look in his signature for the link to his photp page, he's got pictures of the
flattened out xr100 on it.
If i were you I'd keep the kx.
Find a way to de-rate it.
Eric can port those so that the powerband is smoother, and more friendly.
its only 100 dollars(mabye less for a 65), soon your son will be used to his kx, and its capabilities surpass (suspension wise) the xr's.