That's an easy one. My child is harmed because while I'm trying to teach her to use her mind, and she has a most excellent and sharp mind; while I'm trying to teach her to deal with the real world and understand how the real world operates, she's receiving conflicting signals from persons in authority she's been instructed to respect, her teachers.
Thus she recieves, every morning at school, indoctrination from the government that God exists. So instead of teaching her about the right way to get the Charizard to battle the Pikachu on her Ninentdo, I have to take time to explain to her that God is no more real, but a lot less fun, than a Togepi.
Now, you may not consider that teaching my child how best to defeat imaginary monsters is a good use of time. I could be spending the time teaching her about trees, or improving her dodgeball skills, or whatever, that's not the point. It's not your time. It's my time. It's her time.
And any time lost because it's wasted by superstitious belief, is time lost forever, and that's harm. That's real, measurable, harm.
Sure, I have to deprogram her from because she gets infections from her friends who are victims of their own parents superstitions. There's absolutely no reason why I should have to also fight the federal government, not when the federal constitution forbids exactly that kind of establishment.