I have always been a supporter of children reading and enhancing their education. I have over the years found many books as gifts for children in my life on Amazon. They have a great selection for all ages. I have been able to even find books that are hard to find and books that are on their reading programs for school credits.
You can search on Amazon by age group or the great. This is very helpful for a parent or grandparents. We want to support their growth in reading skills. It is hard enough to stay ahead of a child that is encouraged to read. Amazon helps us with this. They even have a lot that you can get on your Kindle.
Listen to your kids and ask what their interests are. It is easy to find a book about whatever they are into.