Thanks for all the tips and ideas. It is an awkward-sized group to split. I think we will try to squeeze everyone in at the same table. I've been staring at the dining room for a while, and I think I can put the high chair in a corner of the room so it's near the table without actually being at the table. Then I'll have DIL sit next to the baby. SS1 will sit next to her. We'll have one chair at each end of the table, three on one side, two on the other, and baby in a corner pointed toward the table. It'll be tight, but I think we can manage.
The baby just turned 1 last month, so she's still pretty little. We're eating late in the afternoon, so well past her nap time. She'll probably be wide awake for dinner. DS is 13, and he's taller than I am, so no little chairs for him anymore!
I think it's a good idea to serve dessert elsewhere. Maybe we'll take pie requests before everyone leaves the table, cut the slices, and serve their dessert plates wherever they get comfortable.
The benches are a really good idea. I'd never thought of that. I don't know if we have time to get benches now, but I'm going to look into that for the future because those would really solve our seating problem. We might be able to get a tight 10 around that table if we had benches. We do have a piano bench!
Thanks again, everyone!