This is for my book I need a way to destroy demons that possess people it needs to be a way that will maim an adult host and kill a child an ideas?

You need to do some research - writers do a lot of that! - and learn all about different types of demons so you can figure out a way to do that on your own. Using someone else's ideas will leave you at risk for getting sued if you sell the story and start making money. HERE is a link to a question about writers and story ideas, too.