For instance, in 2005 the US was one of the last countries to abolish the death penalty for children, not by passing a law, but on the action of the judiciary interpreting a new individual right into the constitution (i.e. to not be executed for crimes committed under the age of 18). Quite how they managed to get the age 18 out of the words "cruel and unusual punishments" is something to ponder.
Prior to that, the states had an inviolable constitutional right to execute children.
So the US was prevented from ratifying the UN Convention of the Rights of the Child because Congress couldn't make a law violating the right of states to execute children, yet the US states could not individually ratify the UNCRC themselves because that would be violating the right of Congress to make treaties.
The outcome of this battle of rights between governmental bodies, is that children in the US are to this day denied individual rights that they would otherwise have under international law, or even national law, and rely on 12 judges to extract their rights from thin air.