I've written this before for someone else, but it works well here as well. My "steps to take if you cannot book seats together".
1) Try calling the airline again. Airlines hold back some seats and may be able to open up enough so that your younger children are beside you.
2) Keep checking - people cancel flights/change seats. Take a pair if you can get it (or a triple). For people in the "single seats", choose ones that other people may want so that you have good seats to trade (i.e. aisle seats near the front of the plane).
3) Do online check in at the 24 hour mark. The seats that are held back are now opened up plus some people may have been upgraded. You have a good chance of moving to seats that are together.
4) Get to the airport early and ask the check-in agent if there are together seats that you can move to (again, people may have moved/upgraded/canceled).
5) When you get to the gate, talk to the gate agent. Changes are still happening and they may be able to help you.
6) If all else fails and you get on the plane and you have all single seats, nicely ask people if they would mind trading (but know that they have every right to reject the request). You have much better odds if you are offering your "better" seat. For example, if one seat is in row 5 and the other seat is in row 32, you'll do much better asking the person beside you in row 32 to move up to row 5 (and you and your child sit in row 32) than the opposite.
7) If you are still separated, you can choose to deplane and take a later flight. Before considering this, make sure you know if there are still together seats available on a subsequent flight (and that there IS a subsequent flight). Also, this will likely be at a cost to you.
I have NEVER seen anyone get to step 7 (not saying it doesn't happen, I've just never seen it) and rarely step 6 (generally only on holiday flights with lots of families none of whom want to sit split up or even go one adult/one child one place and another adult/child somewhere else).