Get the 80/100. My son Gunner is also a large 9 yr old, 5' 2" 100lbs. He was riding a KTM pro sr 2 yrs ago, then he just descided he didn't wan't to ride. I sold it, and then this yr, he wanted to ride again. I bought him a 97 XR-80, which he rode perfect after 1/2 hr in the back yard. He was able to start it himself and even pop mini-wheelies after 1 day. 2 months later, he wanted to take it to the mx-track, and be like "Travis Pastrana". He went out to our local track and tried to jump some little doubles and whoops, go up the back uphill, but kept stalling, and had no power to do anything on the track. I sold the XR, and bought him a 97 CR80, porked out to the hilt, reeds, pipe and silencer, the whole 9 yards. He rides it better and better every time we go. Teach him to go only as fast as he want's to, and what to do if he goes WO. My son is by no means showing any sign of being the next RC, and he actually rides "magoo" most of the time, but is having a blast. He can hardly corner, can't really fan the clutch, just kinda, "bog and zap" on the track, but my point is, don't under-estimate you kids. Granted he has had me telling him what to do the whole time, but he has figured out allot by himself too. The last 2-stroke he rode was a 99 pro-sr, from that to an XR-80 clutch, to a rippin CR80. I thought it would be a long time even before he could really ride or shift the XR, never mind be ready to ride a CR. Good luck, and post back-