Lighter bullets do have lesss recoil. I would still get a 50cal. There are bullets light enough. You can find a load with around 60gr depending on the rifle and projectile that will have plenty of power but not too much recoil. Are you thinking Traditional or Inline? A traditional 50cal with 70gr FFG and a PRB will be light on the recoil side. I personally think a 50cal will be fine with around 60-70 grains of powder and a light bullet below 350gr. A 45cal would be good to but it is hard to find accessories. When she grows older she might want more power and something to elk hunting with.