So I usually start with "what if the back rotors go a little faster?": it tips it forward, beginning a series of operations leading forward motion (like turning the wheel of a car away from center -- what happens if you keep it turned?)
I've also found that words matter: "attitude", "differential", "pitch", "velocity", "acceleration", etc. -- may be hard to access, and simpler terms like "faster", "slower", "speed", "tipped" are usually more broadly understood.
A recent such explanation episode got me thinking more about this - I'd recently got a pair of noise cancelling headphones and wanted to demo it to my kids. In an inspired (or dumb) moment, I introduced it to them like this - "you know headphones play music right, this one plays silence." I'd like to think I caught a whif of both understanding and "what the .." in their expressions :D