Hello, I'm the OP and here is another update (sorry, just slept for 12 hours: jetlag and NIGHT FLIGHT).
We flew BA from Orlando so Mom wasn't on business. She seemed very stressed and managed to wind up little girl by shouting "Are you nervous? Are you nervous?" She went to get a cabin crew lady who came down and told the kids that they could ask her for help, by which time the Mum in front said that she too would help the kids. We never saw Mom again until we were landing ( actually landing with the seatbelt sign on) when she came back, told the kids off for spreading their stuff everywhere, got their bags out of the lockers and... Here's the best bit....
DH said that he would wait with the kids until the plane emptied and she could come down to collect them. Mom said "no" and told the kids to get off the plane themselves and meet them all in the - oh whatsitcalled, that walkway thingy between the plane and the lounge?
We couldn't sit togetther on the way out but were allocated 2x2 seats with 1adult per child but in the same row. Coming back we paid £100 to get 4 seats together (ish as it was a 777 and most of the seats were 3x3. As it was a night flight I sat with our kids so that DH could get some sleep as he had a 4 hour drive from Gatwick to home.
We were in row 30 so pretty far back from the plane. As Mom said, she was 3sets of "plane" to walk through before they got to her.
The son was a bit standoffish but spent the night sleeping on/cuddled up next to DH who fortunately didn't mind.
Get this, Mom was American, kids English. Was she StepMom and only paid business for HER child and HER nanny???? The mind boggles!