What he said. I would also, throw in a 300black out if you can get barnes 110gr ttsx bullets. That is what my 5 year old killed his first with just keep shots inside 100. He has a 243 now at 7. When he starts hunting on his own he wont be able to tote the ARs, but when kids are young AR frame rifles are great sue to adjustable stocks and mile recoil. Bonus points if you have a suppressor.6.5 Grendel, 223 or 243 in adjustable or 12 inch length of pull seem great for those ages.
Adding a limbsaver pad made for AR adjustable stock to an AR-15 in 6.5 Grendel would be my choice if I was buying new today. It would fit anyone reasonable sized also.
I have youth sized 243s and a 7mm-08 micro midas that I have kids use, but I bought em years ago.
Mossberg makes a "Super Bantam" Patriot bolt action that has spacer which allows a 12 or 13 inch length of pull.
I have done the same using blue dot and 125 tnt Speer bullets for .308 win for when the grandkids get older.I downloaded a 30-06 with 110 grain bullets for my grandson at 10 & he was good with it. I’d probably prefer a heavy bolt action in 243, 30-30, or similar low recoil round. Not an option in my state but a can on a 243 would seem ideal.