You're right, it might be better if both characters get some mechanical benefit from the move. At the time, I was thinking that the Kid's benefit was not mechanical, but narrative: somebody's looking out for you, with no strings attached. In Apocalypse World, that's not nothing...
I guess putting both bonuses on the side of the "helping" character was an attempt to really sweeten the deal for PCs to look out for the kid. With my players, a simple +1Hx gets them saying "That'd be nice, I guess. I'll totally think about doing that" while the magic words mark experience never fail to produce a "Hells yeah!"
In retrospect, marking experience should be enough of an incentive. I often find myself adding a +1Hx to moves that help another PC, based on the feel of the Heal/Harm move: When you hurt someone, they see you more clearly. When you heal someone, you see them more clearly.