I don't think that you can really expect them to sit for too long. The baby is still young, so you may want to concentrate on feeding her first and then letting her eat some fruit at the table until she wants down. Our 3yr old would eat with us (we just did our best to have him stay seated) and then when he was done he was completely allowed to get up while we would try and finish. But I do think you kind of have to be flexible and just work at it without making it more stressful.
Family dinner with littles - how do you do it? - Page 3
We start out with a family dinner and if my daughter is not hungry she doesn't eat. She can play around the dinner table and pop back up when she wants.
Sometimes she doesn't eat much, sometimes she eats loads.
If you don't stress about them staying put, or eating a certain amount, then it's not stressful!
If she doesn't eat, she'll eat later or in the morning. Always done it this way and she's perfectly healthy and will try any food. We always enjoy dinner together, even if she's not sitting at the table, but maybe playing on the floor.