You underestimate little kids. *gestures to teletubbies* Also, Zombie Optimus in the end of Dark Awakening(with the way bits of him were falling off) was more disturbing than Headmasters in Rebirth, which is how I assume binary bonding would be used, as there's no need for Targetmasters, and Powermasters wouldn't properly display the partnership that Rescue Bots expresses... though it would be the second most likely. Godmasters wouldn't work because the transtectors don't have personalities of their own(and by the time they do, they don't need their partner anymore) and Brainmasters are effectively Cybertronian Godmasters if you think about how they work.
Oh, and one other example of binary bonding that kids have watched with no problem: Digimon had an entire season/series where the Digimon and the kid combined.