My mum used to write us the occassional bedtime note when we were growing up - they would be intermitant. They might be weekly/monthly or at a special occassion and you wouldn't know about it until you got into bed.
I am staying with my god-daughters soon and will also pop a little note in their bed for them to read - just saying why I love them or what I think of them as a person or why they make me proud.
So what would you put in your bedtime note to your dc?