Make sure to get to the airport early, and you should have no problem.
They hold the 10 (generally) seats by the center exit rows until people get to the airport. Lots of people like those seats as they have a little more legroom. So they ask for them, and that frees up other seats on the plane. (The people seated in those rows have to be over age 15--varies by carrier--and able bodied). Also, you'll have business people with seating assignments who will be no shows. Also, some carriers hold the first five rows in coach for their best frequent fliers, and when they don't get sold they move other pax into them. So a couple hours until fifteen minutes before takeoff they start the juggle to get people seated together.
Here is the bad news though...if you don't have advance seating assignments, you will often be the first people denied boarding (after non-revs) on a sold out flight. They do first have to ask for volunteers, but they do have the right to deny you. If you are denied boarding you have certain rights afforded to you, which vary by how delayed you are getting to your destination.
It is possible that you will not all be seated together, but they will do everything possible to seat at least one of the parents with a child. I don't think I've ever seen a child not seated by at least one parent, but I hve often seen families broken up where the younger kid was with one parent, an older kid and another parent were each by themselves.
The key is to get there early and get those seating assignments!
Anne