"dirty litttle feet" I wouldn't sit down on the ground where those feet had been walking, so I don't like the thought of sitting on the seat where they have been parked.
What the man did was wrong. But, Gononsuch What would YOU have done in the situation? First asked the child to remove his feet from the seat, obviously. I don't know whether other seats were available, but assuming they were not, then do feet qualify for a seat when someone has to stand? And even if there WERE other seats available, then they should not have been on a seat in the seat at all, and I imagine there was a notice somewhere saying so.
Then what? When he refused to move them, would you have walked away meekly?
It would be very useful to know just how to go about sharing public transport amicably with a spoilt and child.