Do you mean you want to make the bags as well as put things in? I would treat them like book bags - the storybook, something like a puzzle to do with the book title, if there are such things a small cuddly toy representative in the story one of my parents made a Cinderella rag doll when it was turned upside down if was Cinders ready for the ball, great stuff! - if you take something such as The Hungry Caterpillar, plenty of resources out there, even objects symbolising the life cycle of a butterfly. You could put in small pots of playdough, small pads of paper, crayons/pencils, seeds, miniature books, travel games, a card game like snap with the characters from the book, back in the day I think we used to put a cassette tape we had recorded of the story - people don't have tape players nowadays😲
Difficult thing can be getting all the resources back! or in good order when they do come back, having the time to check them in and out some type of system would need to be adopted - even if eventually it is "policed" by the parents, i.e. a book where they sign a bag out and back in, will you have a sheet in the bag asking child and parent to assess how the bag helped? What they enjoyed in the bag, what they didn't? Why?
Good luck with it all. If you wanted to make the bags there are plenty online examples on Pinterest.