Ah but you see, you don't feel good because you saved the kid. You feel good because you followed a moral imperative and that was to save the kid. But saving the kid itself did nothing.
We can prove it easily. What if you tried to save the kid but the kid was dead anyway (mauled by a pack of pangolins for instance)? You would still feel good, of course! Because the result of your action is actually independent from your free choice to follow your ethical inclination.