When my boy was 6, I started him off with reduced loads. I did load the minimum load for 125 grain bullets and that's what he practiced with. When we hunted, I used full-power 150 grain loads. He wouldn't know the difference when shooting at a deer.
The next year, he was shooting the full-power loads. Then, at 8, he moved up to 30-06 and has killed deer with that. This spring, at 9, he hit 8 inch target at 300 yards with my 300 Weatherby Mag.
Just start small so they get use to it and not startled by heavy recoil right away.