Several options - none perfect.
1) used XR50 and buy taller bars from Fast 50s or similar. My daughter rode this combination through age 8. She came off a PW50 and the XR taught her how to shift, and to (eventually) transfer braking to the right FOOT and hand. It was a good learning transition (other than wedging the bike under the front bumper of the pickup on the first ride - she just shut it off and left it there).
2) the XR70 is 1 inch longer than the TTR90, but 10 pounds lighter. Also has less power - maybe a concern or not. The above daughter just got a 2001 (found a new carrover for $1049) XR70 and it fits really well. I had a TTR90 in the garage but it definately felt heavier and I was concerned about about the power - I'll bet I've had the 90 rolling close to 50 MPH on the road - too much for my daughter. I sold the TTR to a friend with a son who could man-handle his PW50 like a rag doll.
3) The PW80 and similar suzuki/kawi have a typical two-stroke powerband that's a bit more pipey than the 4-stroke equivalents. Either of these, or the TTR90 could be a good option, though I'd try a throttle limiter to start with. These actually have a little power when compared with the PW50.
