Go to the children's section (but not too close!)
Watch what they pick up. Watch what they read, look at, smile at. Ask for.
Then watch the adults buying children books in the same way.
If you have children you can consult with, or teachers, or mothers, or fathers, or anyone else that has experience with children, get their opinion but more importantly watch what is picked up or bought from the racks or tables.
The picture on the front is very important for the child, the picture and the words for the adult or older child.
You can read about this all you want, but you have to immerse yourself a bit more to get the real sense of it.