To downvoters - I welcome your disapproval like Socrates welcomed his hemlock ;)
I claim that ethics if they exist must be universal.
If ethics are not universal, ie it is right to steal from some but not from others, you will never find a definition of ethics that can be agreed upon by more than 1 person, and their answer will change over time as they rise or sink in power position.
That definition of ethics taken to its logical conclusion is that might = right.
If might is right, then it is right for the strong race to genocide the weak, for the majority to tax the minority, for the master to keep the slave, for a man to rape a woman, and a parent to beat their child.
Opposed to that there are universal ethics - that one rule applies to many parties. For example, the difference in meaning between “receive gift” and “steal” is whether the action is universally ethical. Receiving a gift is voluntary for both parties, and a rule that allows it can thus be universally ethical.
Does this make sense? I can try to give other examples of how this principle makes sex ethical but rape not (do you dare to disagree?), self-defence ethical but assault not, and also how an insurance company collecting payment is ethical, but a government demanding tax is not.
Regarding the claim that even kids understand this: you are right that kids like adult try to get away with not acting according to universalist ethics. But if you speak to them and ask if they think it’s ok that someone takes their toy, they will say no. If you then ask them if it is ok for them to take someone else's toy, they will begrudgingly agree it isn’t. If you ask them what they would think if 5 kids voted and agreed you should give your toy away, they disagree.
Since universalist ethics are true, they intuitively make sense to us, and they also evidently produce the best societies - why comparatively universalist Christendom became the most free and developed place, why land-locked zero-resources relatively stateless Switzerland flourishes while the socialist resource-rich countries suffer in inefficiency and poverty for the masses, etc.