Great info. I can't add much except repeating whats been said. I chose a 20 for my son. A .410 really is a experts gun. I have felt the same way about .243's and young shooters. Which is why I started my son out with a .308. 20 years later. He still shoots the same gun. You might find out when your daughter shoots at game. Recoil might not be a issue. The excitement will probably take over. I could be wrong but your daughter might want to carry her gun. If you end up packing it. She might not be ready. I envy you getting your kids fired up on hunting. Brings back so many great memories. Your going to have a blast!
A youth model 870 is a fairly light gun. At 8 my son packed it around fairly well, and he was a small kid. I'm not too sure what your plans are, but with a 20 your daughter will be ready for geese, and turkey, and have slug options for deer. Just a thought.