Same here. There was the flight that we took (booked only 2 weeks ahead of the trip) where we had four separate seats. They had my DH in one row, DD8 separate a couple rows behind him, DS age 9 months was in a completely separate part of the plane and I was about 4 rows away from DS.
When we got to the airport, I knew they were going to have to do something, because there was little likelyhood that some stranger was going to want to sit next to and deal with my 9 month old for me. Sure, they might not have been HAPPY that their seat assignment was changed, but really....
It turned out to be no problem at all. I didn't care if all four of us were together or not, as long as each child had a parent in the same row. Anything else would have been irresponsible (and I imagine would have been a liability issue for the airline to leave a small child with no parent there - imagine if one of them had a peanut allergy and didn't tell the flight attendant and got peanuts?). DH sat with DD and I sat with DS a few rows away, and we didn't mind that at all.
The 9 month old would be a problem (if he was in a car seat, the airline has to make someone move - the only time that they do). Otherwise, no, there is no liability issue. Children as young as 5 can fly without their parents on the plane at all; an 8 year old can certainly fly seated a few rows from a parent (you make no mention of any disability).
In this specific case, any irresponsibility was YOURS. You booked your tickets knowing that all that was available was four separate seats. If that was not acceptable for you, you should not have booked that flight.