My go to caliber has been the .243 for deer, pigs, antelope, varmints since I was a kid. and my 9 year old daughter has it now. I just picked up a new one for my son recently
If I knew I was hunting Bear I would carry my 7 Mag. The only time my .270 or 7 mag come out of the safe is when I am taking someone on a pig hunt that has never killed a big game animal. I like the added energy from the larger bullets in case the shot is not placed perfectly.
for me the .243 youth rifle with a detachable mag is a perfect truck gun.
That being said, when I first looked at a bigger caliber than the .243 I did a ton of research on ballistics, energy, trajectory, etc...I refused to buy a 30-06 and went with the 7mag. I love it and have never regretted the decision.
now, since then I have come to this realization, the 30-06 has put more meat on the table than any cartridge out there. you can not go wrong with a 270, 7mm, or 30-06. ammo is readily available at every place that sells it. Look online at all of the possibilities for different grain bullets.