It's been said already but I'll add a story, one that my hunting buddy related to me. He was a lefty, and had a Savage in 243 that he rebarreled to 260. His load was nothing special, 120 gr sierra Spitzer with 47.5 gr of H380. I have a box of his ammo. One night we were swapping lies and laughing when he tells me, "You wouldn't believe how that 260 smacks deer, it's one heck of a caliber."
My buddy had hunted whitetails for years with the 243, he was a deadly shot, and still, he and I tracked lots of his game, so much so that I don't think it's a good deer caliber. The 243 is a 22-250 on steroids. One heck of a decent varmint caliber, and the 6mm Creedmoor with probably un-throne it. When Warren Page was doing his 6mm development he tested a few shoulder angles and his version had a steeper angled shoulder on the same length body, which gave the cartridge a longer neck. To get onto the 6mm bandwagon Winchester simply necked the 308 case down to 6mm with the same shoulder and called it the 243. It never was ideal, but it beat the 244 Remington, it's only competition. There is lots of knowledge out there about case length, shoulder angle, neck length and ballistics, and today the 243 just misses the mark in so many ways. Not all bad, just better stuff out there.
The 6.5 Creedmoor is in essence a 260 Remington with a few issues corrected. It is an engineered cartridge, it's time is today. Interestingly, to work it needs a faster than the normal 1:10 or 1:12 stuff of the last century. It also needs good bullets, better than was the norm years ago. It will and does work well.
Youth calibers: Regrettably I don't see many calibers engineered specifically for youth or small hunters. Some of the older ones would qualify in my book. The 250-3000 comes to mind, or the 7x30 Waters, or the 25-35 Winchester in a single shot or light lever. They book at close to 243 ballistics, but they use the increased caliber to their advantage. Lower velocity allows for reliable expansion with normal bullets (cup and core), and with less powder, they don't bark and buck like a higher intensity caliber.
When I hear 6mm or 243 talk I remember the words of EK after shooting a running whitetail with the then new 244 Remington, "Ain't much of a caliber is it.". And the other EK gem, " I prefer to hunt for my game before I shoot it".
I ramble, my bad, but just the other day my son and I were caliber talking. We each have a Ruger M77 Mark II ultralight in 223, and if you load that with a 70 gr Speer, it would make one nice deer gun. Not long range, but within 30-30 range, it may be the ticket.
No matter what you choose, use a good bullet, a good bullet in a bad caliber is better than a bad bullet in a good caliber.