boy/girl sleepover

hi my daughter is 11 and her best friend is a 12yr boy, they are very close like brother and sister, they have sleep overs in both our house and his, they sleep in seperate beds in the same room with the door open, both myself and my husband are always in the house and his mother is always in his house, they are very innocent and i know nothing "grown up" is happening...i love the fact they are best friends, i think its teaching them both its ok to be friends with the opposite sex.....naturally as they get older they will have to stay in seperate rooms, but for know i think its fine, have other parents come across this? id appreciate others thoughts on this,