For children and those who are extra skittish, I want to give a nod to the .22 short. It gives almost no kick, and if you are wearing good hearing protection, you still have to pay attention to *hear* it. It is a great icebreaker for those above.
It is an excellent bullet for the inexperienced to learn proper handling of a rifle.
An important twist is that it is actually a frustrating ammo, another good effect, in that even novices want to step up to a stronger round. A .22 LR is gratifying after that.
A good step up from BB and pellet shooting.
Another similar option is .22 CB Long, which is closer to .22 Long Rifle in length, so it will generally feed reliably from .22 LR mags (but not cycle the action on semi-autos).
For training someone who is close to adult size, a pistol caliber carbine is an alternative that also offers lower recoil & noise levels, and relatively inexpensive ammo (during 'normal' times ;^); plus, a PCC works for basic self-defense functions, especially for the gun owner who just wants "one gun"...