Ill throw out what I did for my kiddo.
He started with an 22lr at 5yo.
Was shooting an ar15 55gr fmj around 7yo.
Started on a ruger american gen 1 in 6cm at 9yo. He was shooting 75gr v max downloaded with h4895 2900fps ish. WITH A SUPRESSOR and a bag of sand in the buttstock.
With that he was able to shoot sub moa 5 shots out to 400 with the occasional flyer.
At 10 with his first 2 elk tags in his pocket he is shooting the 103eldx at 3000fps. He shot his bull on his 10th birthday and has a cow hunt in December.
He put 3 shots in the bull in about 15 seconds at 320yds. Never lost the bull in the scope and shot a 1.5moa group at his first big game animal.
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This would be very hard for a kid to do with a larger chambering. If you are a reloader you can make your own reduced recoil loads and make the best learning experience possible.
Get a gun the kid wants to shoot hundreds of rounds a year.
Then take them shooting!