Agree. I'd have shifted them and wiped down.

I'm assuming you wiped the seat you finally sat in so no difference.

Unless on at first stop there's no way to know no one's sat there and still no guarantee it had been cleaned first.

I wouldn’t , like many of the other precautions you have to assume people generally behave sensibly or be incredibly paranoid. You don’t know the person who serves you in a shop has washed their hands either and could drive yourself mad assuming the worst.
In this case the kids were clambering everywhere so it was reasonable to assume they’d done the same before boarding the train so higher risk.