Thank goodness all was well for you xxx
I swapped seats recently with a young child so they could be with family. Whilst only a short haul from Miami to MCO on a 3-3 formation flight with AA, I found myself sat next to two young girls, across the aisle from my DH, DD & DS, I soon realised that the two girls sat next to me had their parents behind and another poor sibling sat between two strange men several rows away. I asked if the crying girl was their sister which was confirmed and said I would swap - the little girl could not thank me enough.
To be fair - I was not sure if the family thought they HAD to sit in their allocated seats, as I found it strange that the youngest member of their party (she was about 6 or 7) was the one separated. Why one of the older members of the party did not sit there ... but hey... I had just spent 9 hours flying next to my family from Heathrow to Miami - I was happy to have a break from them on the connecting flight!!
We have been to Orlando 6 times now with First Choice, Virgin, BA x 22, AA & Delta and have always been able to choose seats way in advance without any cost and never been separated. Now our kids are 13/15 I am less concerned but I really can sympathise - I would have been equally frantic,