Well I've had a variety of opinions here but I pulled the trigger... I ended up getting my son the Trek 820 (26 wheels) in a 13 inch frame but I did replace the tires with smoother running Serfas Drifter City tires. My son is 4'9" and a pretty strong soccer kid and although the frame to his body looks a bit big his leg extension with the seat set properly is great and he has no problem controlling it at a stop. Bottom line is he *loves* it and wanted it strongly over the 24 inch kids Trek Superfly. He will grow even better into it soon and if I'm being honest we aren't a high mileage family so the whole "he will get tired on a long ride" doesn't apply so much, plus he had better cardio than his parents LOL. I didn't honestly think too much about a youth version of the Trek DS (26 inch wheels) which perhaps I should have since it seems like it would be perfect, but my son also likes that he is the only one in the family with a "mountain bike".
My 13 year daughter who is barely under 5'1" got a new 2019 Trek DS2 Women's (what used to be called the Neko) out of the deal too and she also loves her bike. It fits her very well and may well end up being the right size for her even as an adult.
My real problem? My low mileage 2012 Trek 8.4 DS (in white) looks lame compared to the new neon yellow/green 2019 Trek DS3 with its new frame design (new for 2019, much cleaner and modern looking) and integrated cable paths etc. I have absolutely no reason to get it AT ALL but damn I want it!