I am now 72 years old and have used a 20 ga. shotgun since about 13 years old. My dad used a 12 ga. pump for squirrels and rabbits. You are fine, just follow the advice of these other guys. Don't shoot them too close unless they are very close and the head shot is there and use smaller #6 shot. I don't kill many squirrels these days but back when young they were part of our freezer stash for the winter. Make it fun for her, you won't regret it.