Loxley - do you want the full details of our trip? It consisted of 7 legs altogether; all but one of them we were seated together. My husband called up a couple of days before to confirm our flights, and we were told all was fine. So far, so diligent of us.
We arrived 2.5 hours before departure to check in, to be told that the two airlines (in partnership to cover the entirety of our flights) hadn't joined things up properly, and so we weren't technically on the flight. We spent a fraught 2 hours trying to resolve it, for it to be sorted out at the very last minute, boarding cards for the 3 legs issued, and we literally ran through security and the airport to make the flight on time. All 4 seats together. Great.
Got to the stop-over, and went to board the next leg, and realised the problem.
My DH spoke with the travel agent this morning, and they take full responsibility. Yes, we should have checked to make sure no mistakes were made, clearly we were at fault there, but, well, we're not perfect.
We were travelling for 44 hours door-to-door, including three flights, with a 3YO and a 5YO, and yes, we dropped the ball by not noticing someone else's mistake in time. It's a stressful situation at the best of times (i.e. no DC), but especially so when travel agents can't even get basic things right.
Regardless, we had to deal with it, when we realised it. Young DC can't be expected to travel alone for such a long flight, and other passangers wouldn't have appreciated the situation either. Well done, if you've never made such a mistake in a similar situation.