Re: caliber choice for 6 year old son?
A lightly loaded 243 with a medium contour 20" barrel and cut down stock would be a sweet learner. The second one could be a .284 , 6.5x284 etc. I'd set the second action aside and in a few years, let him decide (it will mean that much more to a 12yr old, getting to decide on his own rifle!).
If some is good and more is better, then too much is just right.
My mind is a raging torrent, flooded with rivulets of thought, cascading into a waterfall of creative alternatives