I took my 4-year-old to his first games last year at UCLA and at SDSU. For the UCLA game he lasted almost until halftime and again almost through the third quarter. Then he wanted to play with the binoculars, his cousins, and the goo underneath the seats (nearly making his 7-months-pregnant mommy add her own special blend to that goo). We kept his attention by helping him with cheers, knowing when to clap, and by teaching him the numbers of some of the key players (Kehl, Hall, Collie, Allen) and asking him questions about the game. He had a good but not great performance.
By the time the SDSU contest rolled around, he was pretty attentive for the most of game. That stadium is a great place to bring a kid because it's empty enough for him to move around a bit, but full enough to keep him somewhat corralled. He played around during the timeouts and such, but would reengage when it was important.
This year we probably won't be able to hit any games (living in Southern California), but we're going to keep up his education with the distance learning program of televised games.
Good luck to your sons. "Train up a child in the way he should go..." and all that.