At 210 lbs I would not recommend the KX125. You would have to rev the guts out of it to make it go.
There are a ton of mods out there to make the KDX anything you want it to be but stock they are pretty mellow- and quiet. (Good for the back yard) One warning about the 220 is the stock pistons have a habit of breaking, most people replace it with a Wiesco.
Compared to the KDX the KTM is lighter, faster but lacking the cushy seat.
Probably the most important thing that you will need to do with a two stroke is make sure that they are jetted properly as you will be putting around for a while as the kids learn and don't want to be fouling spark plugs.
There is a ton of info on this site about KDX's and
www.ktmtalk.com has a ton of info on KTM's.
And welcome back to riding!