I think it would depend on what you are teaching them.
Anyone teaching their kids that 2 + 2 = 5, which is the equivalent of teaching a 6000 year universe or a literal Flood, should have their kids taken into care.
If they said they 'believe' the universe is 6000 years old that would be fine, but when they brainwash kids then the gove has to step in.
Christians would be the first to condemn a cult that uses underhand techniques to get a grip on their next recruit (and victim). Yet many Christians do exactly the same as modern day cults.
Prey on the vulnerable for example, a technique of many cults, and you don't get much more vulnerable than a starving child in an African village, a prime target for Christian missionaries.