The thing about the 308 is that you can buy dang near ANY type of ammo imaginable! Once you start to reload, the sky is the limit! Hodgdon - The Gun Powder People has some YOUTH reloading information using H4895 powder that will turn a 308 into a genuine ***** cat. I would use the 135gr SMK bullets with with mentioned H4895 loading information. The resulting load will be truly pleasant to learn with. Later on you can scale that loading up as your wife begins to get more comfortable. My 12 year old ( at the time ) sister learned to shoot with a 308 using the above mentioned reloading information. She had a Savage 10FP 20" barrel rifle.
Best of luck sir!
Gary
P.S. The other reason I chose 308 is the drastic barrel life improvement over a .243.