Ever been to a live wrestling match in the late 70's to mid 80's, before wrestling was made for TV entertainment, where the "bad guys" literally were hated and the fans in the first 10 rows were filthy, toothless drunks? The Mexican fans are those first ten rows of wrestling, times 100! You and your 12 year old should be more than fine, as I assume you will be sitting with a predominately American crowd, and there should be more than enough time to leave the stadium before the Mexico/Panama kickoff. If the Mexicans had the first game, I would suggest not taking your 12 year old, especially if Mexico were to lose a close game or there was a controversial call that went against Mexico.
If you stay and watch the 2nd game, don't be surprised to see cups of piss, golf balls, sneakers, D batteries, and anything else that can be brought in and thrown. I wouldn't be surprised if boat or road flares were set off. I, personally, would bring my 12 year old and leave after the first game.