Ah, so she only assumes your a Satanist. Well, I guess I'm not surprised by this, given her fanatic Christian beliefs.
My advice, go about your business as you see fit. If this place is really public, then she can't say anything about who goes there or what not on the basis of her assumptions. She can assume anything she wishes, but I doubt any legal action against you can be taken on her mere guessing of your religious affiliation. If she doesn't want you around her children, well then that's another matter. I suppose legal action of some kind could be taken in this case, though I am not sure of German law. I was just thinking of a restraining order or something, though still, this seems quite unlikely given that she has no real reasons to base such a legal action on.
I'd say that if she doesn't want you to help her children, then don't. Allow the children to rebel against their mother, for I'm sure they'd have more sway in the matter. If the children seriously wish for your help, then they will do whatever they can to get you back. I have no idea who these children are, or what their personalities are like, though it seems they see you for you and don't really give a damn about who or what you believe in. Let them rebel for you if need be.
I ask that you take my words with consideration. I try to give the best advice I can given what I know of the situation, and I do not claim to be correct in any of this. I do wish to aid you if I can, however, so I have replied to your post. If you take anything from this, great. If not, so be it. In any case, think it through and don't act irrationally.
Love. Laugh. Live. Forever.
"There are no coincidences...only the illusion of coincidence"--V for Vendetta