Welcome to FT. Seating changes happen and they suck especially when flying with children. Most of us have learned to check on seating assignments regularly as sometime changes are made without any notification. Even for flights that are months out.
Your best course of action is to keep checking on seats. For the return flight get to the airport a bit early to make sure each child has an adult in the seat next to them (if that is still not the case). There are requirements about that. That said I do find it odd that even with a change of equipment you lost your C+ seats. Unless the change resulted in a reduction of C+ seats. Which does happen.