Durandal has posted an excellent article about the most simple of all devotions, that to the Blessed Virgin. A 1 minute prayer recited daily is protection against the wickedness and snares of the devil. When my daughter at 4, would have intense nightmares to the point that I believed her to be tormented by an evil force I told her to recite a Hail Mary and sleep with her Rosary around her wrist the nightmares ceased. This is the Prayer that children should be taught first after the Glory Be...
I fly to my Mother, Virgin of Virgins,
Pray for me O Holy Mother of God.