Admittedly most airlines use the approach to run an overnight job and sweep pax out of the elite prebook sections. UA does this for E+ and others do it for their elite prebook sections. From an IT perspective, the airlines built a simple overnight batch job to run and audit the prebook process. This is the 50 cent solution.

Also, it is possible that the OP booked the seats and then got reallocated the family to row 31 after the night job had run. Did ther OP ever check the seat allocations between the time of booking and OLCI?

I also submit that the reallocation at least resulted in the family being given seats together, this will commute the possible compensation. IT not as if they were scattered throughout the cabin in middle seats.