I agree with britz about the muzzel brake...noise (even with muffs and plugs) causeds a lot of flinching. I never believed it until I was stnding over the shoulder of my wife shooting a pistol with a brake...WOW! When I was concentrating on shooting it I never noticed it, after I knew about it I couldn't get past it.
If you are going with a brake, a 260, 270, 308 shouldn't be a problem. I have four kids, 2 boys and 2 girls who all started on a 6mm Rem 788 with about 2 inches of stock cut off. None of them had a problem shooting the 6mm without a brake. IMO the 6mm/243 is the hammer of Thor in deer sized game. I would find a good Remington in that caliber have the stock cut to fit, find a good reload (I've had great luck shooting with the Remington cor-lock bulk bullets) and let her kill anything that moves and shoot anything that doesn't.
Whatever you do, you are already going in the right direction. Good on ya dad. I love seeing people get their kids (especially girls) out shooting/hunting. I've missed a lot of prime fishing time due to soccer games and lots of missed opportunities on deer and elk trying to get my kids on the animals. I guess saying I missed the opportunities isn't correct; I didn't miss it a bit. I got a bigger kick out of my daughter shooting a doe this year than any animal I have ever shot.
Genises 27-3: Now therefore take, I pray thee, thy weapons, thy quiver and thy bow, and go out to the field, and take me some venison...