Yes, paying for allocated seating is an annoying expense which shouldnt be an additional charge.
But if I was flying with DC, I would do everything in my power to guarantee we would be together. For lots of reasons, but preomindantly for their safety, my peace of mind, and so as not to distress them or inconvenience anyone else. If I paid to guarantee my seating arrangements, i.e. sit with my DC, then I would not move for someone else.
DH and I are going on holiday just the two of us later in the year. We dont care where we sit. We will try and sit together, but will happily move if someone else has a greater need to the seat we end up with.
But no way would I take a chance on not being seated with DC, or make assumptions about the airlines or fellow passengers' willingness to move to accommodate mine and my children's needs. Your kids, your responsibility to ensure the seating will suit you.
The system may be wrong, but it is what it is. In my opinion, you cough up or take your chances. If you dont like it, dont fly.