My oldest son, who's normally more cautious than his younger brothers, @ 15 YO went from an XR200 to a 250 2-strk (RMX); quite a jump, but he did the transition by taking it easy until accustomed to the (dramatically different & superior) power & suspension, both of which increasd his confidence to a point where he now races enduros alongside me. He learned a few things the hard way, like "think smart or don't do it" big-time hill climbing, dealing with real brakes for a change, & after trying an XR400 last spring said he's glad I didn't get him an XR.
Although the XR250 is a fine all-day scoot & knarley/survivor-trail bike that can be raced, I'd suggest a KDX instead as it packs a little more punch (or low-hour, older but post '90 250MXer with a fly-wheel weight).