if you are flying with children under a certain age (i think it might be 12) legally they have to sit with an adult that they are travelling with, they will move other passengers if they have to.
if you are very worried about it just check in at the airport early (ok so no one likes being at the airport for a long time, but it is worth it for the piece of mind, and hey what else are you going to be doing) and you should be able to get seats next to each other.
if you book by phone they will also be able to allocate your seats, even if you cant on-line anymore