Should children of people who belong to cult organisations like westboro baptist church, KKK or MAGA be removed for their own safety?

Good question... On one hand that will only alienate and escalate the parents further, and perhaps the children as well if the indoctrination already prepared them for that possibility. On the other hand, they are definitely in danger.

And where do you draw the line? Do you stop after MAGA? Move on to all Catholics, all religious people, flat earthers, antivaxxers?

Yes, definitely

Not sure that this is the right path. But parents who indoctrinate their children with these ideologies are vermin.
I do not think MAGA is anywhere near as bad as Westboro or KKK.
Should not all US citizens strive to make America great again?
Is KKK still active?

You're going down a slippery slope here.

But you really think Trumpers should have their kids taken off them?