The best rifle for a smaller child would probably be an AR-15 in 5.56. The low-recoil from the light round, the recoil-slowing gas-operated semi-automatic action, the adjustable length of pull, and the simple practicality of shooting from a bi-pod or tri-pod as with a Hog or Pig saddle, make it ideal for adapting to young children (like 10 or 11 years old). The straight-stock and pistol grip are also ideal for shooting prone or from the bench where most children would start (as opposed to shooting off-hand standing or kneeling).
For an older boy interested in hunting, an AR chambered in 6.5 Grendel is ideal. There are other suitable chamberings. Other popular ones are .450 Bushmaster and .300 Blackout -- all very good for deer, but the Grendel is going to have the most versatility since it can reach out far beyond hunting ranges and also shoots flattest within them.
For bolt-actions, check out the CZ 527 and the Howa mini-action. These are chambered in .223, 6.5 Grendel, 7.62x39, as well as some other small cartridges. Length of pull is a problem because unlike with common AR furniture, it's typically not adjustable. You either have to chop the stock or buy an adjustable one. I bought a Boyds "At-One" adjustable stock for a CZ 527. It can be adjusted quickly to fit either of my boys or myself although it is a bit short for my rather long LoP (there's another insert available with a longer range of adjustment). For the Howa mini bolt-actions, there are several chassis available that allow adjustable stocks similar to AR or precision-rifle styles.
In my opinion, these are the best options available for youth rifles without a more specific purpose specified. None of the other rifles are better. The .243 and 6.5 Creedmoor have longer-actions, more weight, balance farther forward, and produce excessive recoil with more costly ammo and offer no benefit to kids for any purpose unless they're shooting elk-size game or farther than 1100 yards.