Kids that age have amazing memories. My son got his tech and general when he was 11. I taught him about waves and reflections using water analogies, but most of it he did by grinding through the online practice tests.
Don't worry too much about getting them to understand the theory. If they stick with the hobby the theory will come in time. Just make sure they know the rules and procedures enough to stay out of trouble.
Remember that by memorizing the correct test answers they really are learning some things, especially when it comes to the rules.
Here's some (hopefully) useful tips:
I set up my O-scope, signal generator and a small loud speaker. I let him play with the frequency and amplitude so he could see and hear sine waves.
Then I connected the speaker to the O-scope so it worked like a microphone. I got a wine glass and some tubular wind chimes and let him ring them and see the waves. One point I emphasized is nature likes to make smooth sine waves. I had him visualize the waves in a fish tank you see from the side, and waves from a pebble thrown in a calm pond. They all look like what's on the o-scope.
I had him visualize a narrow channel of water opening out into a lake. Drop a rock in the beginning of the channel and the waves go down the channel and out into the lake. Block or just partially block the opening to the lake and the waves come back. Most kids can visualize that.
The best moment was when we were at a neighbor's house who had their pool cover on. The cover was held down by long water bags. My son stepped on one end of a bag and watched a wave go down the length of the bag, then return back. "Hey dad! Just like in ham radio!"