First, I have no doubt that AA staff will help you on the day of departure. There is NO way they will allow a 2 and 4 year old to sit away from their parents..... and I would refuse to fly if they tried.....
Try to check in online as soon as you can to see of any seats together opened up... if they did, snag them. If not, get to the airport as early as possible and detail your situation with the gate agents.
As a last resort, as the flight attendants to help you.