Like just about everyone else, I think you are a little lean on meat. Rule of thumb is about a 1/2 pound of meat per adult. Depending on the age of the children, 2 children count as 1 adult.
I also think appetizers will help. Fill them up before dinner and they won't eat as much.
You could also make a simple dish of baked Rigatoni to help supplement the dinner. Just take a pound of hamburg (browned) and mix it with some sauce (If using jarred/canned sauce, I suggest about 2 jars/cans). Cook 1.5 pounds of Rigatoni (do not over cook), mix it all together in a large casserole dish and bake it (covered with foil) for about 45 minutes at 325. When it's done, heavily sprinkle some Parmigiano cheese over the top and let the heat from the Rigatoni melt the cheese. You could mix this all up the day before then just bake it on the day of the dinner party to heat it up. Depending on the age of the kids they will probably like it much more then the pork loin and the adults will like it too. The whole Rigatoni dish shouldn't cost much more then about $10 depending on prices where you live.
Good luck on your party and have a very Merry Christmas.