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?
I left DD a little note once saying I hoped she had a lovely day the next day (I was going to be at work all day), and that there were 4 kisses because she was 4. She was ridiculously pleased with it and kept it stuck to her door for months.