I agree with Markus, we really need more info on your kid to make a good reccomendation. My grandson is 10 and just over 5' at about 110-120#. He has enough reach and is big enough to shoot an adult size shotgun. For a kid that size, I would recommend the TDC in 20ga. It has choke tubes so you can shoot just about anything with it. It has a vent rib so you can mount a set of clamp-on sights for deer hunting. I guess the new ones are coming with 28"barrels like a 12ga, but you might still be able to find NIB one of the earlier 26" barreled guns.
If you are dead set on a dedicated slug gun...2 choices for a kid...the Tracker II 20ga($140 or so) and that comes with open sights, and the USH 20ga Youth($160 or so) and that comes with a scope base and 22" heavy barrel.
Thats my 2 cents, since you ask. My grandson's first will be TDC just because it will broaden the options for him, since he will only have 1, for now.

cookiemann