sasrocks:
Sounds like I am a few years ahead of you dealing with kids and bikes. I recently found a KDX 220 for my 12-year old (my boys are big and tall).
We went through the full course of bikes, starting with XR50s (now called CRF50s), XR70s (still auto-clutch), XR 80 (first hand-clutch bikes), then did quite a bit of looking for the next step. My boys tried numerous bikes out, including KX 65s, CR80/85 both standard and big-wheel versions, KX100s and the TTR-125s. Interestingly, neither of my boys liked the 2-stroke bikes. We ride most trails, including the open desert, forest trails, single-track and rocky washes. The 2-stroke "hit" is difficult to control on loose scrabble. The mini 2-strokes have little torque, which means the rider absolutely has to manage the clutch and RPMs.
We ended up getting both a TTR-125 and a TTR-125L. By the time we were through, the L model had the full-line of BBR modifications, including a header and muffler, airbox mods, skid plates, BBR springs front and rear, triple-clamps and TAG bars, carb mods, shark-fin, chain guide etc. The TTR's were great. They are vary easy to ride with a wide torque band, especially with the airbox/pipe mods. The TTRs also seem to be practically indestructable. The only real maintenance I did was regular oil changes and air filter cleaning. Both kids loved their TTRs. The younger kid got the hand-me-down L model, and was very reluctant to give it up.
The only down-side to the TTR was peer pressure. Here in So Cal most of the kids who ride do so at the motocross tracks, which means they are constantly exposed to the motocross hype. All the "too-cool" motocross rats really look down on the TTRs. If you are not on a mini-pumpkin, a YZ85 or a KX85, you are pond scum. For kids, the cool factor can be very important. The motocross bike industry thrives by bombarding consumers with propaganda convincng them that anything less than a Ricky Carmichael replica racer complete with every conceivable (and some inconceivable) modification is inadequate. Kids are particularly susceptable to advertising and hype, and that is what drives the industry. My older boy took a lot of heat at school about his TTR. The fact that when we rode with other kids on their full-blown motocrossers he regularly kicked their a**s only made matters worse. All of this may not be a factor where you live, but you may want to talk to your kid before plunking down a few grand or more on a bike.
That said, a friend has a KX100 that has been a very dependable, low maintenance machine. Any of the small 2-strokes will eventually need top ends, but the KX 100 seems to go a lot longer between overhauls than the 80/85s or the 125s in our crowd. As the 100 does not seem to fit into any racing class, most of the used ones you see have been used for trail riding, which is a big plus . The only drawback is the small tank/short range. I don't think there are any desert tanks available for the KX100, but one manufacturer makes a front number-plate/auxillary fuel tank that works well. I think it hold about a half-gallon or more of fuel.
While I'm writing a novel, you didn't say how big your 11-year old is. If he is growing as fast as you can shovel fuel into his pie-hole like mine do, you will probably be looking for the nest-next bike as soon as you buy a current next-bike. With that in mind, I suggest that you don't get caught in the trap of trying to find him the "ultimate" bike for now, just find him something serviceable and that you will be able to re-sell in a year or so. If a particular bike is highly desirable now, it will probably retain some of that appeal when it comes time to sell.
My bottom line is to find a bike that is fun, acceptably cool, fun, low maintenance, fun, that allows some growing room, and that is fun.
We have blown through thousands and thousands of dollars on bikes, boots helmets etc, and all of it has been some of the best money I have ever spent in terms of family bonding and all that other mushy, touchy-feely stuff. The only thing that really hurt was watching my wife bring home her brand-new KTM400 and park it next to my very well used '96 XR400.
Some photos the famil & bikes over the years:
XR70 (now he rides a KDX220)
XR70, XR80 and Mrs 4Rider on her XR250
Youngest kid on his TTR125L
Youngest on his KDX220 a week or two ago
14-year old on his XR 400R
Wife & Her EXC:
Whatever you do keep riding with the kid/kids for as long as you can.