The fact remains that the pamphlet was given out to children. Even if they could read it, would they even understand it? I doubt it. Most kids young enough to trick or treat are still under belief by what their parents believe, it usually isn't until they are older that they begin to question their parents.
If my kids received, say, a Hindu pamphlet or something I may even find it interesting. If my daughter read it, she would show it to me and ask about it. (I don't have to worry too much for my boys, they are both special needs even though both can read- one well the other not too well
), I would tell her that it is another religion/faith, who have a somewhat different idea of God than we do. (I would never tell her it is a false religion or anything since I don't believe that).
So, although it may be annoying, I don't think it will cause any harm.