Well we booked our holiday a month in advance and when we pre booked our seats at the time of booking the holiday there was none together for the return journey. So If you’re booking a relatively last minute holiday this could happen to anyone.
However, I wouldn’t expect anyone who had paid extra for their seats to move.
myself and DS3 were sat next to an elderly man and I was hoping he’d offer to swap with my husband in front so we could all sit together. If I was in his position there’s not a chance in hell I’d sit next to someone else’s toddler if I didn’t have to. However he didn’t, and that’s fine. We managed. We swapped seats half way through.