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.