Re: caliber choice for 6 year old son?
For a six year old to shoot any more than one or two shots with I'd think that even the .243 win could be a bit too much, you really don't want to make it anything but fun for such a youngster.
My recommendation would be a 6PPC with a 22" - 24" bull profile barrel. I have owned and regularly shot with such a rifle for the last 14 years, killing deer and pigs out to 500 yards with the tiny 70gn ballistic tips.
You get almost no recoil, good barrel life, it's easy to reload for, produces incredible accuracy, has great case life, just enough power for deer sized game and it's murder on coyotes and varmints.