Nap Time Play Date

Upon entering this scene, I said, "This doesn't look like nap time."

My five year old responded, "It's a rest time-play date!"

They were loving Little Golden Books that were kept in a lovely, old suitcase at Grandma and Grandpa's.  M was reading to her brother. What's a mom to do?

Can someone please explain to me how it is that my kids get along so perfectly during-but only during-nap times??