So let me understand. You think he should have immediately taken the coach seats. In the next paragraph you say you have asked people in the past to change seats as the OP did. So for others they must follow the rules to the letter, but for you it is OK to bend the rules?
That's called double standard.
I understand your frustration, but honestly you can't just expect others to move for you just because you are traveling with a child. The GA offered you seats together - In coach. I hate to be harsh on this, but if you wanted seats together you should have taken the coach seats.
I sometimes ask others to switch seats if I am traveling with a companion, and usually people are willing. I only do it if I am switching for "equal" seats. Like aisle for aisle or bulkhead for bulkhead. I don't want to put another pax in a situation where they chose a aisle in row two and I ask them to go to a window in a bulkhead. I think that is rude.