Do your best to get checked in ASAP and see what is available.
If there are no seats together, choose the best seats you can, even if they aren't anywhere near each other. (i.e. aisle seats first choice, window second, front of plane) That way you have a decent seat to trade with. It is much easier to ask the person in the aisle next to your daughter's middle seat to trade his aisle seat for your aisle seat, rather than ask someone in a nice aisle seat to move elsewhere and be stuck in the middle. Or, if you can offer someone in a middle seat your aisle seat, usually they are thrilled to move.
Then, alert the counter agent or gate agent as soon as you can to your situation. They often can move someone and get you squared away. If not, they are often very helpful in alerting the flight attendant, who can help find someone to trade with you on board. In general, people are helpful about moving so a mom can be by her two year old.
I have been there, done that, many times. I always pay to get seats together, but it's not always possible. The unknown is always stressful, but it seems to work out every time.
Good luck!!!