RE: .223 for 8 yr. old for deer?
If you're set on taking her...which it seems you are...What about an in-line muzzleloader. The modern ones have almost no recoil and are deadly as hell past 100yds. You can even load it a little light and practice to make the perfect load. It's great fun, and she will begin to understand how the bullet actually gets the power to leave the barrel.
Just a thought.