Childrens book help!?

Perhaps you can illustrate this concept with images of a store and things that children like. For example, consider a video game or a certain type of candy. If every child has one, then the demand is low and stores that have this item in supply will be left with too much on their hands. However, if few children have the toy or candy and the stores have a lot, the supply will be high and the demand will be high. On the other hand, when the stores a little of the item and lots of children want it, the demand will be high and some children will be disappointed when they don't get it.

Hope this helps.

Maybe just make it more simplistic? Like a little girl selling cookies at 25 cents a cookie, then theres all of a sudden a cookie shortage, or people just really start to like them and she raises the price so she can afford more cookies for more people and afford her pony and her castle lol

Use your imagination, you'll think of something. Just remember to keep it simple. Simple but entertaining. Maybe even a little funny.