depends on the room she is in ... for that matter it depends on her mood also..
20 is a good choice but don't be afraid to let her try a 12 with a low brass 2 3/4" bird shot. Women actually shot 12's just fine if they step up to them. There is usually a skeet range around that will let you rent guns.
if getting the 20 get a compact size and consider a collapsible stock. Her first instinct will be to find cover (as it should be). Once behind the couch, edge of the bed, or in the closet, she will need to easily be able to maneuver he gun.
Revolver are also good choices as there is just less to remember. Point and pull trigger. but the lack of safety makes it an issue with kiddos.