What about the other countries that have signed the convention?
I'm certain they were NOT concerned that the UN would tell them how to take care of the children. Unless, of course they were not behaving accordingly.
They signed it because they want an international agreement about the matter, making it easier, for instance, for separated parents from different countries, to insure the best education and best care to their children. Children are by essence, innocent and must be respected.
The countries, as Tom suggested, that might not apply the convention, even after they signed it, can then be shown the convention and force their courts (or international courts) to apply it. If it's still not the case, it becomes a political issue and it's up to those countries to figure out what's their best interest.

I've just heard that the reason why the US has not ratified the convention has to do with the punishment (in case of crime or other deeds) inflicted to minor children, that's quite disproportionate, relative to what the convention wants to apply.

Raoul