K.K.
Hello, Have you tried going onto Amazon.com yet? I have an account with them and when I want to get new books or games (I just received the Kindle Fire after having the old Kindle) I go there and type in Free Books, Free Games, etc. I think you might be able to type in Free Childrens Books and find something. I know that there were some classic books that popped up when I looked last. They might be more geared towards your older child, but I also got a free book for my three year old granddaughter recently onto my Kindle and then transfered it to my new one when I got it.
Good luck.
K. K.