One 13 x 9 pan would make 6 large servings (one cut lengthwise, then cut into thirds across the short way). You could get 8 servings by cutting into fourths across the short way. These would be decent-sized pieces for most appetites.
You could make 9 pieces by cutting into thirds along the long way, but they would be much smaller pieces. You would need to have several side dishes: maybe a big salad and some good garlic bread, plus a dessert or fruit?
It really depends on what size servings people expect. Since three of them are children, you probably could make it work.
I don't know anything about how to figure out timing for two pans. But I think two 13 X 9 pans would be too much, even for 9 adults.
DS '04 "Boogaboo"
DD '08 "Lilybear"