Red Dot? Primary Arms M3 clone or M4. Low cost, great quality.
Distance for .22? Depends on what you want to shoot. If your plinking with your son, like I do, we zero at 50 yards (he is using the Ruger SR 22 with the red dot, I use a Ruger 10/22 with a simple 4 x 32 fixed).
It's fun to see the cans jump or ballons pop...25-50 yards is good..can always move out further as he (I) progresses.