Help the parents deal with the decision of buying a book and also with questions that the child might ask or that the parents might want to raise as the child is reading through the book.
Publish a short guide for parents about the book. They can peruse it in the store to help them decide on buying the book and take it home for free with the book they bought to help them as their child reads it.
If your store caters to kids, make their shopping experience a delight, please their parents and have a chance to sell even more.
Offer a service to the parents where they order the kid’s present in advance. You gift wrap it for them and hide it somewhere in the store. You also write the kids name or otherwise mark their gift wrapped package with ink only visible in UV light. When they arrive, you give the child a magic wand (UV torch), and they go through the store searching for their hidden treasure.
If they want to do it, they might as well do it the right way and with supervision.
A book of fascinating but slightly dangerous experiments that kids would love to do. The book explains the tests, the danger involved and give exact measures to protect the kids and prevent the danger from happening.
Teach kids business and entrepreneurship in a fun way.
A business selling “franchise” kits to parents and their kids. These include teaching materials on how to build the business: basic definitions, functions, how to start, how to plan, marketing materials and principles, etc. Different “franchises” can be bought, for particular age groups and interests.