I've mostly had reasonable experiences with Delta when traveling with children. In the half-dozen times or so our pre-assigned seats have gone up in smoke and we've been separated, the gate agent has taken care of it before we've boarded, whether we've been in coach or Business Elite. I do believe that young children ought to be entitled to sit next to a family member, and it's the airline's responsibility to figure it out. Which Delta seems to do, though apparently mainly for its passengers with elite status.