We had great luck with my son on a TTR-90. It was the perfect bike for him at the time. Easy to use 3 sp auto, decent enough suspension for the trail riding we did, ran well, started easy (this was before the magic button was available). Mellow power delivery (my kid was not at least then showing any signs of becoming a mini motocrosser wonder kid!). We managed to even do some 40 plus mile days in the trails with the 90. I also could have some fun messing around on!
I think the key is knowing your kid - how agressive of a kid is he? Will he develop real quick as a faster learner physically oriented kid...and quickly max out the suspension capabilities of the bike? (that was not the case with my son) Is he in the midst of growth spurt so size may be a concern? I have found buying a bike where at first it is a wee bit of a stretch size wise, then milking it out at the end where he is perhaps a bit on the to big side for it keeps the cost down. A hidden advantage of him being on the big size for a while is they then can really begin to feel the "master" of the bike - a real confidence booster!
Oh...dont forget to consider dealer support. If you got a good relationship with one brand dealer or another...I dont think you could go wrong with any of the ones you suggested. (for me no one is local so it didnt really matter!)
Now my kid is on a TTR-125L. Again...it was a perfect choice for us. Started out a tad on the big side last spring, and will by this fall be a tad on the small size (yikes...only one season...he is only 11 and now wears a size 9.5 boot! - that kid better stop growing!). I really like playing on his 125 now...fun little pit bike!