Hey Fishhead -
Just wondering how your son did with the KX-85?
I'm in a similar situation... here's the deal:
1) My son is turning 10 in a few weeks. He's 4'8" and 70lbs. Slightly tall for his age, but thin and lanky.
2) Coming off an XR-70. Previous to that he was on an XR-50. I'd like to get a bike that lasts more than 1 year.
3) We ride strictly off-road, woods, trails, single track. The trails have a fair amount of sand for a kid.
4) He's not a super aggressive rider, but is cautiously improving his riding skills.
5) Watching him ride from behind on the XR-70, I think the smaller bike, small tires, low-footpegs, and hunched over the handlebars standing up is holding back his skills somewhat. He is definitely ready for a new bike this season.
I too have looked at the TTR-125 (small wheel version). The bike fits him great, but at 176lbs, it weighs 50lbs more than his XR-70! Also, I want to wean him off the tame power of small, heavy four-strokes, but there aren't a whole lot of small, woods-friendly 2 stroke choices out there.
Seems like the KX-100 can be made into a great woods bike, but it is just way too big for him (we tried one on for size last weekend).
I have the opportunity to buy a used, recently rebuilt 2000 KX-80 that has KX-100 cylinder on it. It's been ported and tuned for a woods-friendly low-end powerband. It also has KX-100 wheels on it, so essentially it's much closer to a KX-100 than a KX-80.
My son threw a leg over a KX-85, and while somewhat tall, it was doable. The great thing is that it's only about 15lbs heavier than the XR-70, 11lbs lighter than an XR-80, and a whopping 33lbs lighter than a TTR-125.
If your kid's experience with the KX-85 has been good, I'm thinking about getting the hybrid KX80/100, and putting the smaller KX-80 wheels back on it. When my son is tall enough, I'll put the larger KX-100 wheels back on, and presto, he has a larger bike to ride for another year or two.