Well since this thread got bumped...
My 9y.o. son is too damned big for his kx65, so my 7y.o. gets to move to the "race bike" from his drz110. I didn't think he'd do as well on another 2 stroke as even though he's not afraid of it, he never put all of the 65's power to use. I also refused to buy some overweight, under suspended, and under braked trail bike (like our heavily modified 110... still a turd), even though he is a trail rider. I've got a few super mini sized (19" front, 16" rear wheel) conversion bikes (drz125 with rm85 suspension and brakes, klx140 with 170cc big bore and kx100 suspension) but they're too big for him still. Here in California, any competition bike after '02 gets a "red sticker" which severely limits when and where it can be ridden, so I couldn't bring myself to buy a crf150r.
A couple months ago, after some thought, my wife made one phone call and we were on our way to pick up her old bike, a Dave Miller Concepts (DMC) built XR100 in a CR85 chassis. It's seriously the perfect playbike with the handling of an MX bike, and the easy to ride power of an xr, and it retains a "green sticker" so he can ride it year 'round. It's got both little (17/14) and big (19/16) wheels so he can grow into it, might even last him long enough to get him to full size bikes. With the built (120cc, Web cam, 200x carb, DMC exhaust) xr engine, it's supposed to make 11-12hp which is enough to have fun on a bike this size (a stock xr100 makes around 6hp, ttr/klx/drz 125 are around 7-8hp) without having the light switch like powerband of a small bore/non-power valve 2 stroke. Dave Miller passed away earlier this year so it's kind of a special bike to me. Service Honda (now defunct) sold a similarly equipped (engine work, suspension sprung/valved, billet clamps, asv levers, fatbars, etc) bike called the Junior-Y 100 for over $9k, our's is still in near new condition for less than 1/4 that price. Happy to have it back!
That got me in the mood to buy more goodies for my playbike (the worked over drz125) so I bought some RSW triple clamps and taller bar mounts, Odi fatbars, rebuild parts for my rm85 forks including heavier springs, and a few other odds and ends. I have a 150cc big bore for it, but I'm really considering ponying up for an Engines Only 190cc build (21+hp!) We'll have to see if/when I'm able to ride again, I've had a rough few years with my health and just had spinal fusion in June. For a guy who can't ride, I sure have a lot of motorcycles!