My kids are 10 and 9 now and we've only just started reading from the OT with them. We stayed away from the very difficult parts of the bible and I stuck with the parables of Jesus, they LOVED the story of the miraculous feediing of the thousands and anything Jesus taught. I never spent a lot of money on bible story books and such. I mainly trusted God to provide what we needed to teach them. We'd pick treasures up at thrift stores and discount bible story books. Now that they are older we do bible study, reading every night. We just read thru the bible. We've been thru genesis recently and have moved on to exodus now, with a bit of the book of John thrown in there. You honestly dont have to spend a lot of money, just some prayer time

. God will lead you.