This just happened to me. Booked for my 2 children (8 and 3) and myself to fly from Australia to the UK by BA. Read the T&C re. seat allocation, families will be seated together, allocation 5d prior to travel. So checked via My Booking on the BA website, and yes, we were sort of together - they were in one row, and I was in the row behind. A block, yes. But not ACTUALLY together, in any way in which it mattered.
Of course I could have waited for online check in to open, 24h prior to flying - but then, given the vagaries of the fucking BA website, and its propensity for crashing, this would have been extremely stressful and probably ended up with us not sitting together, for 22 fucking hours.
SO I felt forced into paying the extra money to get seats together. It's not as if the plane was already blocked out - ALL the rows in the last section of the plane, bar the exit seats (which we're not allowed anyway) and the bulkhead seats (which are used for families with under 2s) were available, at a cost. So actually, BA could have chosen to put is in a window row of 3 - but they didn't.
Luckily, they did see sense on the way back, so I only had to pay in one direction, thank goodness! But I would have paid it again if they'd separated me from the DC again. 22h with 2 small DC by oneself is quite stressful enough, without being out of arm's reach of them!