Keep checking the seat map, and see if other seats show up open. If that doesn't work, once you are at the airport ask the gate agent if they can reshuffle people around. Sometimes they can. If all else fails, you will need to throw yourself on the mercy of your fellow flyers.
Where are your seats located--window, middle, or aisle? I'm hoping you are not all middle seats. Try to keep the least desirable seat for yourself, and use the best seat to trade with. For example, if you are in a middle seat in row 27 and your child is in an aisle or window seat in row 5, ask if someone next to you would mind moving up in the plane and taking your childs aisle seat. It is usually easier to get people to move towards the front of the plane than it is to get them to move towards the back.
If you end up having to ask people to switch seats, please do it as nicely as possible, and maybe offer to buy them a drink. There are people who have posted here that they just plunk their child down and say something along the lines of "If you won't switch seats with me I hope you have fun keeping my child entertained. Oh, and little Johnny also pukes in flight. Have a ball". That is so not what to do!
Keep checking the flights first. You may get lucky. Other than that, work with the gate agent, and your fellow travelors as a last resort. Good luck!