Something tells me that you'll probably get a note from your teacher if your son or daughter talks about writhing serpents killing the innocent to a class full of small children. Thankfully, fantasy doesn't have to just be about the killing of or by mythical creatures. There are many books written specifically for younger children.
Harry Potter starts out kid friendly and actually grows in maturity with your children. They are light and fun for the first couple and grow increasingly dark as the series goes on. Alice in Wonderland and Grimm Fairy Tales are both fantasy books and stories that will help expand your child's imagination. When you get right down to it, isn't that the purpose of fantasy. It allows you to wrap your mind and imagination around concepts and creatures that could never exist in this world.
It lets them gain a appreciation for what does exist and believe that anything is possible. Fantasy is more than just a good story, it's about creating a world that has never existed and giving it life in your head. Encourage your children to read fantasy, but make sure its appropriate for their age. No parent wants to get that phone call from the school. (Believe me, I know.)