If you want to make a YouTube video of her getting hit in the face, then a .44 Magnum or bigger handgun, or a 12 gauge with 3 1/2" shells and only a pistol grip, are the way to go.
If you want them to have fun and possibly want to do it again, then a scoped Ruger 10/22 (or insert favorite semi or bolt .22LR repeater here). More accurate than a pistol, safer to use (harder to swing it around to ask a question, etc.), and can be a lot of fun to dump a mag at 25m or 50m into some swinging targets. Scope it as they're older and more likely to have problems seeing iron sights.
Besides, you mean to tell us you have no reason to own a 10/22? That's almost like saying you don't have a reason to own a computer or a refrigerator.