I strongly disagree with the policy. If it's dangerous for a two yr old then it's dangerous for an infant as well. Two make the mandate apply specifically at an age known as "terrible" (twos), seems pathetically ridiculous.
That said, basics in conflict resolution is to de-escalate. Explaining to the person in control why they're wrong and your right is the exact opposite.
My two yr old would never keep a mask on for 30 seconds let alone a whole flight, but I wouldn't argue with the FA. I would apologize and stick it on his face knowing full well that he's going to rip it off in a minute. I would do that every time it was requested.
I would also try and shield him from view of the aisle by sitting him in the window seat and trying to entertain him facing the window.