Alliegid, don't be an offensive ignoramus- there are NO Satanic connections with Samhain ( the correct name for Halloween if you are a Pagan- and it is a Pagan festival). Pagan's do not even believe in your Devil (only you Christians have him as your bogeyman) so how could it possibly have any Satanic connection? If someone believes in Satan it means that loosely they are playing by Christian rules not Pagan ones, so any daft individual who is practising Satanism is in fact embroiled in your doctrines not ours.
Samhain is a festival where we believe that the veil is thinnest and that a reconnection can be made with loved ones and those we have lost. It is a time for reflection and for understanding our place and the place of life and death in the greater scheme of things, it's not evil, it's not wrong and if you don't mind me saying so it's offensive of you to suggest it is, I can't imagine you would be so rude about a Sikh or perhaps a Hindu festival, yet with this you seem to feel free.
To address the poster's question, most people are not Pagans, so this is a harmless 'Disney' style party your little one wants to go to, nothing more, I doubt the 6 year olds have any religious overtone to their little fun day, so I would personally let her enjoy herself as it doesn't clash with your beliefs at all if there is no religious element.