Those kids grow up to be Batman-style superheroes.
The kids who were home-invaded are pretty much average ordinary people. Though their parents are dead, they never forget the hero that saved them from the same fate. They would want to be just like him one day. They would be to David Dunn what Batman would be to Superman.
Joseph will get superpowers.
Because he only has half of David's genes, he would get some powers but not all of them.
David has Superspeed.
One of the things that was talked about alot in the film, was David's reputation as a former football player. To be a great football player, you'd need strength, speed and endurance. Since David was considered a star player and has been proven to have strength and invulnerability. Maybe he's a fast runner too thanks to his strength.