Being good and working in your own self interest aren't mutually exclusive though. In fact most of good things are genuinely done out of complete self interest, and practically all good things on the corporate or bureaucratic level. There's always someone involved in the decision who directly benefits, luckily sometimes it's all/most of us too. Society gets better when the self interested goals of an individual are aligned best with the majority of people (e.g. why presidents are on average better than kings).

To give a very active example, Zuckerberg isn't throwing billions into making ML models and then giving them out for free out of some sense of cosmic right, he's doing so to eliminate competition and further Meta's goals of making more money. But since his actions align with what most of us want, i.e. open source models, it's all seen as a relatively good thing. There is no such thing as "objective good" in nature, there's just how actions influence the person doing the moralizing.

Every living being on earth is hardwired to do two things first and foremost: survive, and make sure its species survives. The ones that weren't are simply no longer around. Helping others is ultimately something you do out of self interest, since you benefit from it indirectly. It is absolutely the natural state of things.

Children aren't tricked into believing it because it's actually true. As you point out, people are inherently good, and they act against their self-interest all the time. It's always a mistake though. ;D