As a kid I loved Halloween and trick or treating. It never meant anything more to me then costume and candy and frankly it doesn’t mean more to me now. We are not “celebrating” Halloween.
We have to pick our battles!
If I was to forbid them at this point it would be a major punishment and I see no reasons to do that apart from the fact that I really do not like them to eat a lot of candy. I usually end up throwing a lot of it away.
They know that all the ghost and scary stuff are evil spirits and they are not interested in that aspect. I do not let them wear scary costume (not even see pictures of their cousin who gets a worst costume each year).
I know I am a minority here obviously!