Sorry to say this Annoyed321, but your DH is an imbecile and a twit. I've booked with this website a few times before and their online check in is extremely straightforward. I suspect he was very late logging in and therefore made some excuse. There is NEVER a problem with seating a family of four together, or a child with one parent (2 separate pairs), or two children with their mum (3 together). I strongly suspect he messed up the booking or tried to save something like £30 so that he could only check in late and have to accept that everybody will be scattered far and wide. If I'd messed up myself I would be asking my spouse to come and take a look and help, not hide it from them. But then again I don't cut corners for holidays abroad.
If there is a seating issue, it is often worth asking the staff at check in (10% chance of getting seats together), if unsuccessful, asking the staff at the gate (5% chance of getting seats together usually if there are no shows - ie people have checked in online but don't turn up), or ask the cabin crew very nicely, calmly and politely upon boarding- the final say rests with the onboard supervisor , who is the most senior flight attendant on duty during that flight. Be patient, be prepared to stand with kids quietly and unobtrusively to other passengers while they discuss where everyone is. I've been asked to swop seats before- included a coveted aisle I got up early to reserve! - but always by the flight attendant, never by passengers. Once, their intervention was essential - someone had been assigned my seat! Well we can't both spend the flight in the same seat. The man stayed in his seat so I went to tell a flight attendant I'd found a problem and explained it politely and calmly. He said, could you wait a while so that we can sort it out? I did, while still holding on to my hand luggage. (He didn't tell me what he was going to do). After what seemed like a long but not unreasonably long time, I thought I would be put into some cramped corner seat, but he said "please follow me", and led me to business class! (I felt like I didn't totally deserve as the other passenger had been more punctual and boarded first). But they said they reseated me and not him because I had my luggage with me and was polite and ready to move (the other passenger had already put his hand luggage in the overhead compartment and settled in comfortably!)
This only works if the flight has a business or club class that isn't full. If you're on one of those charter flights or a full economy class-only "low cost" (I use quotation marks as budget airlines can sometimes be more expensive than BA!). It's an awful situation if you are faced with a plane that is full and you can't find two or three decent human beings not travelling with children or disabled companions willing to be civilised and move around to help out some kids. Usually getting your DD or DS to cry loudly and unendingly on cue upon sitting down is effective- the noise generally makes the adults on either side so agitated that they will beg you and your other DC to take their seats!
I'm usually a control freak over the booking and would have refused to leave on holiday if I wasn't in charge of the booking myself.
If faced with the situation of being separated on the flight like you were and there really was no other convenient solution, I would have told the crew I'm not flying, asked to offload our checked in luggage, and either booked another flight or stayed home. If I did have a traumatic personal history as you have with SA, I'd have absolutely refused to go on the flight. (But would have tried getting both kids to cry loudly first. If bribes/rewards to encourage them to give their best Oscar-winning performance didn't work, I would really have given them something to cry about, eg tell them some fake bad news that would trigger the tears.)
Your DH is useless and awful. I'd have got off the plane with the kids and be preparing to give him the boot. (Sometimes, however, if you ask to offload, and leave the flight, the cabin crew miraculously manage to get people out of their seats to swop with you. Because offloading means they could be at risk of delaying the flight and being fined heavily. But I have only ever flown with airlines that have never allowed a seating situation like that to get out of hand.
The cabin crew really do have the final say - that's part of aviation law - and not passengers regardless of whether they have paid more for early check in or seats with their BFF or newly wed spouse or whatever. So when it comes down to it, it was actually the cabin crew at fault here for your misery. And as a parent, I'd say a 13 year old does not need to sit next to their parent/guardian, but for safety, eg turbulence, accidents, etc all under ten year olds should sit with their parent/guardian. Which awful airline or holiday company (if it was a charter flight) was this??