We had busy bags at our old church, they were filled with age appropriate things, these were for a bit older kids so they had things like little books with bible verses, and stickers, and a note pad with crayons or pencils.
The idea behind it is whats so cute, you fill them with whatever will keep the kid "busy" while you put a baby to bed, or make lunch, or just need the kids to stay in one spot for a bit. You could do board books and such, hang them up and when each child wakes up from nap (while the others are sleeping) he/she could make a habit of grabbing a busy bag and sitting in their own area in the living room to explore their bag quietly until the others are awake. Cute thought.