I believe your kids will be able to fly through the AoPS Pre Algebra in no time.
My son finished 5th grade math in 3rd grade. He just finished 4th grade this year with the regular curriculum (probably not a good idea). We also went through the AoPS Pre Algebra book and videos this year. We also had a tutor once a week for general Q&A if needed.
He also got his feet wet on competitions. He LOVED it. Kids like him, where have they been? You could not wipe the grin off his face.
I also stuck a couple questions from the AMC8 on his school folder several times a week. Just for something to think about. these are great questions. They seem to have a range of questions for every level.
thanks for starting this thread.