The big issue here isn't so much the amount of recoil but the size of the gun if you are trying to get a 5 or 6 year old outfitted. You are going to have to get a youth stock for whatever you choose and the stock on the full sized Marlin is going to be way too long for her to shoulder. I'm in the same boat and at the present time looking towards a youth Handi-Rifle in 44 Magnum. You get the short stock and smaller size reciever that's easier for a kid to carry. Since I reload, the 44 Magnum offers a huge variation from 800 ft-sec loads to full house magnums. I already have cases, dies, bullet molds, etc for the cartridge also.
I may change my mind a dozen more times though before I actually buy something.