As cabin crew I have had this issue on board and I have been happy to let the child keep the ear defenders on. I would rather have a calm child with calm parent who is there to assist them in any emergencies, than a flustered parent possibly having to physically restrain a very upset child. Most of the time the child is below the seat level so you don't get other pax going "but how come he's allowed to wear headphones..."
I think for the good of everyone a little common sense is sometimes needed, rather than blindly adhering to the letter of the law.