Abuse is not protected by the UDHR just because it's part of someones religion, that's just nonsense, the right to exercise your religion ends when it involves violating someone elses rights. The reason indoctrination is not illegal is that it's not seen as child abuse and the reason it's not seen as child abuse is because if it were you would be treading on some very dangerous ground.
After all they are technically teaching kids what they view as being the truth about the world so to make that illegal you would have to define what is or isn't fact and what can or can't be taught to children. Now that in itself is impossible to probably do or at least enforce, but even if it was doable, you would have to have someone somewhere define and enforce the "truth" which is incredibly dangerous thing to be doing.
@Mr Tree Why limit it to religion? Why not beliefs in conspiracy theories like the world leaders are lizards and we didn't go to the moon, or flat earthers? Religions aren't the only irrational beliefs that children are taught or indoctrinated into believing, there's a whole list of things you could ban if you want to go down that road, but as I said above the "truth" would ultimately have to be defined by someone somewhere and that is and incredibly dangerous amount of power to give them.
The best we can realistically do is put into law a minimum for what has to be taught to children and hope that that gives them the tools to still think for themselves and fight indoctrination.