She wanted to treat him to a bedtime story as part of his birthday present. She said to him "this is the first time since you've been a grown up that you have had a story read to you by a kid!". I love these moments.
Also, here is a joke Lily made up today: why do grown ups groan? Because they are GROWN ups.
I pointed out that she could ask "why do adults groan?" because the words are synonymous to make it a bit more of a surprise and she thought that was a good idea. :)