I would pretty much echo what everyone else said. The key word is Trail/Play bike though. Most 2 strokers are MX oriented and it's harder to dumb one down that to spruce a KDX up.
#1 Weight, I haven't met a 4 stroke yet where I'd put up with the weight, except maybe a YZ 426, but there I won't put up with the starting woes, and again that's a whole nother beast :D and not a 'Play bike'.
#2 Ease of maintenance. Somewhat, I know of XR's that have had nothing but plugs put in them for a few years now. But two strokes are simpler mechanically,
#3 This site & forum. Not to be over looked. I'm sure the quality of information on other parts of DRN are good, but the information you'll find here on KDX's alone is amazing.
#4 KickStand, sure you can put one on an MX'r and a lot of 4 strokes have them and a lot of people take them off. But for starting out, and definatly if you are riding with/behind kids, you want a kick stand. Kids seem to crash where there are no trees or anything to lean a bike against, at least mine do :)
#5 Putt-Putt abilitiy. You have that with a four stroke, but not on a lot of two strokes. You do with the KDX.
-MD