I had a mom ask me to give up my isle seat in exchange for a middle seat so she could sit next to her kid. I told her no. I hate the middle seat. She rolled her eyes at me which validated my decision.
You made the right call. I’ve done the same. It amazes me sometimes how stand-bys or other late flying cheap ticket holders want someone else who specifically planned to keep out of a middle seat to accommodate them because they are ‘family.’
They’re on the same plane for a trip that doesn’t last days and weeks where they’ll never see each other.