I agree with the other posters so far.
I was born and raised in the evangelical Protestant church, and as you know, those kids learn the Bible. Most evangelicals, including the children, can find Bible verses while Catholics are stumbling through the Table of Contents to find out where the book of Habakkuk is.
The children's teaching concept that you have to grasp is out of the Bible and found in Isaiah 28: 10--"For He says, 'Order on order, line on line, a little here, a little there..."
Other translations say, "Precept upon precept."
First, there's a big difference between the brain and heart of a 5-year old vs. a 7-year-old vs. a 9-year-old. These kids should be separate "classes." The 5-year old is not ready to learn what the others are ready to learn.
Secondly, the main thing that children of these ages need to learn from the Bible is timeline and stories with names and events, not details and concepts. Take them on a general walk through the Bible, Old and New Testament, and make sure they know all the Bible stories. There's no need to use a child's Bible story book. Use the Bible. YOU read through the stories and prepare them for the children.
One group that has done this beautifully that you might want to check out is Child Evangelisim Fellowship. Yes, it's Protestant. But when it comes to telling Bible stories without "dumbing things down," they are the masters. I taught the stories of David to a class one year, and I had to work my tail off to study the CEF materials and correlate them with the actual Bible. They truly cover it all at a children's level, and they also adapt the stories to be told in multi-age settings. Perhaps you can find a Good News Club in your city/town and take your children (You go too). These clubs teach the entire Bible over a period of several years.
Did you catch that? Several YEARS. CEF has been doing child evangelicism and catechesis for many decades, and they use the "line on line, precept upon precept" principle. Not too much all at once for children.
Another group that I think does a good job with children's Bible curricular is Pioneer Clubs (until recently, these rivalled the AWANA clubs, which I do NOT recommend because of the watered-down Bible lessons that emphasize rote memorization and include no application at all of the various lessons). Pioneer Clubs material also includes an "Application" of the Bible stories and lessons, but it is very simple and adapted for CHILDREN. E.g., out of the story of Sodom and Gomorrah (which I don't believe PC covers, BTW), they would apply the principle of obeying God, even when we want to do something else. Even a child can understand the concept of obedience to God and parents.
I don't want to sound lecture-y here, but I am 53 years old and have successfully raised two daughters, and I've taught thousands of children when I was Protestant, and I was a popular and successful teacher. What I find is that sometimes, parents are convinced that their children are much more advanced than all the other children, and so they try to introduce them to adult curricula and rush them along.
Certainly some children are advanced and highly-intelligent, especially children who are raised in homeschools where both parents and other relatives are involved in their education. But children are still children when it comes to the matters of the heart and soul--they need to be taught spirituality "gradually," precept upon precept, starting with the simplest of stories and working their way up to the deep concepts and applications.
We need to be very careful to protect the innocence of our children. Just because a child understand calculus or chemistry at age nine doesn't mean that he/she is ready for homosexuality and the destruction of cities and taking of human lives by a loving but just God and the Old Testament culture that led to the incest between Lot and his daughters. This is all hard enough for adults to understand, let alone children. Build up all the other precepts first, and don't be afraid to use those "baby Bible books." Baby steps, like the Bible says---line on line.