I've shot a lot of ducks with a 20 and a few geese. It will work but you have to be aware that a 20 doesn't have near the number of pellets that a 12 does. So, you don't have as much room for error. When I was shooting geese with a 20 I didn't take long shots and I tried for the head/neck area. The question is, at 11 years old can you wait for the close shots and can you pull off the head shots on geese? Only you can answer that.

As to shooting one sitting in the dekes. I wouldn't do it. not only does it go against my sense of sportsmanship but good dekes are getting expensive and I don't want mine all shot up. But to each his own.