How hard could it be for the flight attendant to ask the person sitting in the same row with the parents to switch seats?
I didn't know that, but what parent would let a three year old sit alone on a flight. The child should have been by mom or dad, and the other parent sitting alone. That would have been the easiest and made the most sense. Parents should be in charge of doing what's right for their child, not a flight attendant. If the seating is as you say, it should have been a no-brainer for the parents that one of them should move.
How hard could it be for one of the parents to switch seats with the child? I'm not giving up my aisle seat so the little family, who obviously didn't plan ahead, can sit together.
My reservations are usually made months ahead and I request an aisle seat. I check the website seating chart ahead of time to make sure I have an aisle seat and I show up at the airport early in case there is a problem, so it can be corrected before all the seats are assigned. These people could have done the same thing to insure they were seated together. The only reason I can think of that one of them didn't sit beside their child is that their two seats were in an exit row, but you usually have to ask for those and rarely get them randomly.
Whatever seating issues they had it doesn't matter in the end because their child wouldn't stay buckled down in a seat and they weren't unfairly removed from the plane, so they need to just shut up and get over it.