I'd suggest staying away from all the new super slugs and just going with a plain old 2 3/4" rifled slug, or reduced recoil slug. In a youth sized 20 gauge, a flatter shooting slug that hits harder at 150 yards isn't going to gain you any real advantage. It probably doesn't have good enough sites to let you judge holdover and make accurate shots at longer ranges anyhow.
To top it off, the recoil might not be enough to stop you from shooting, but it will probably be hard to control. I think this is even more important with a single shot, as you already have more time involved in a follow up shot. And instead of only having to get the gun back on target, you have to have the presence of mind to get the gun open, loaded and closed, and then back on target. The last thing you want in an emergency situation, is to be trying to do that with a gun that rocks you back a step and makes your arm go numb.
Another thing to think about is what choke your barrel is. I know its fixed, and thats why its important. Its not really recommended to shoot slugs through a full choke.
You might look into a carbine in a pistol caliber. A lever action in .357 or .44 mag would be much better for the role you're wanting to fill, and might work out real well if you have a side arm in the same caliber.