Personally I think a 243 winchester is too much gun for such a wee lassie, she will develop a flinch that will impede her future shooting. I would be looking at a 222 /223 to start with and as she develops she can go from there. If you must stick with the 243 use lighter bullets and drop the charge to the minimium stated in your reloading data. If You stick with the faster burning powder you can go lower. 3031 is a good powder or Hodgson equivilent
Slow burning powders do not like reduced charges, its dangerous.
Also Have a Pachmayr Decelerator Recoil pad fitted Decelerator is a special material to absorb recoil and lasts longer than normal rubber pads
Ear Plug Hearing protection will also help her if in the field ,
Vaughn G in OZ