This a favorite of ours and so easy. It's great for those evenings when schedules get crazy and kids are eating at different times.
1 jar of your favorite spaghetti sauce
meatballs (I buy the frozen bagged kind)
Pour the jar of sauce into a saucepan and heat up the meatballs on low till they are heated through. Place as many meatballs as you like onto the bread (I estimate about 6 small meatballs per person), along with some sauce. I use anything from a fresh French bread loaf to (desperation) hotdog buns. Sometimes I toast the buns if they are too soft so they don't get soggy.
Top with mozzarella cheese. Leaving the sandwiches opened, I put them in a 350 degree oven till the mozzarella cheese is melted and browning just a bit and they are all warm and toasty.