CMMG and Aero Precision both make complete guns in 6.5 Gendel, and they're both very solid companies. If you wanted to save some cash, you could get a cheap complete Palmetto State Armory lower, and a complete CMMG or Aero upper.
When buying complete ARs, quality tends to match directly to price until you get up to about $1500, where, while still getting improving quality, it's not as much extra quality per dollar you spend after that rough price-point. Definitely diminishing returns. The heart of reliability in an AR is a well-built bolt-carrier group, and properly assembled gas system, so buying a higher quality upper gets you extra reliability and generally also better accuracy as well.
If you're looking for something bigger than .223 or .22 cal, you might also consider 6mm ARC - it's a great option, with a bit less recoil and a bit flatter shooting than 6.5G.
Given that this is a gun for your kids to learn on, one extra bit that I can offer is that muzzle blast is often just as much of a factor in developing a flinch as recoil, and a suppressor slashes both pretty well.
