To avoid the comments like "I don't want to wipe everyone's noses" or "they don't want me trying to talk to them like they are a child", let's imagine that the "own child" is your child, but a fully grown adult. Or, we could just imagine that they are any of your close kin, whom you care for and want to be happy.
It isn't that they have any specific qualities similar to a specific type of kin, but just that you have a natural empathy towards them and wouldn't deliberately harm them or even take advantage of their weaknesses for your own gain.
An advantage it has over the golden rule is that many people either don't love themselves or come up with dishonest claims that they would want to be duped if they were dumb enough to be duped, thus they have an excuse to dupe others. I call bullshit that anyone wants others to take advantage of them in moments of irrationality (we all have them), but shifting it from oneself to "someone you love" blocks this type of excuse.
It means that we can still treat people conditionally upon how they treat us and their decency toward others, but the starting baseline of how we treat them and the benefit of the doubt we give them is higher. It means that unless a specific person gives you strong cause, you cannot dupe them in an economic exchange under the excuse that "they're dumb enough to fall for it". It is most relevant for limiting the actions we take that could harm others, which includes convincing them to do things against their own interests. IOW, it makes it just as immoral to lie to your sister about the condition of my car when I sell it to her as it would to lie to my own sister.
The biggest weakness I see with this perspective is in areas where we do not act to go out of our way to protect people against other people or events. If my mother is sick, I am more obligated to help her than is a total stranger. It makes sense that should be the way it is. Acting to harm is immoral, even toward a stranger, but failing to act to prevent harm that you didn't cause is not automatically immoral, but can be if you have some personal connection because looking out for each other is part of the expectations of personal relationships.
To fix the weakness, we could change the rule to "Treat everyone like they are your best friend's cousin whom you do not know but they care deeply about."
It obligates you not to act to cause harm or even take knowing advantage of them, but doesn't obligate you to be the one that acts to protect them from other harms that you are not the cause of.
