It happened to us once. A family of 4 scattered throughout the first class. My very young son, about 9, politely asked his seat mate if he would change seats with his older brother, about 14. The gentleman was charmed by his manners and maturity. The seat swap happened before my husband and I had a chance to ask anyone. He told me later that his plan B was to ask his brother's seat mate.
I believe most flyers are accommodating if seat type is identical and request is done respectfully. Also a lot of flyers would rather a kid to be seated with his family.