I mentioned before that my younger boy has fun playing D&D, but he really would rather play a game with guns and cars and smart phones. So last year I got a POD copy of d20 Modern.
He never had the attention span to actually sit through the rules (not that I expected him to at only 8 years old), but recently we made characters and have been playing very fast, very loose impromptu games when he has the interest (pretty much every day) and I have the time and energy to run it for him (not every day, unfortunately).
Stevie made a Fast Hero with the Law Enforcement background, so a cop. But he spends as much time committing crimes as he does stopping them. So fairly realistic.
I created a Strong Hero with the Adventurer background, a martial artist to be his buddy (not a cop).
At first, when we got the book and I explained the Wealth mechanic, he was upset. He wanted to be able to total up lots of cash on his character sheet, and buy lots of stuff. Now that we've played it a bit, he really digs the Wealth mechanic. He can buy anything he wants of value under his score, so as it goes up, he's constantly adding more stuff to his sheet. And he keeps trying to buy big ticket items with wealth rolls, too, so his score fluctuates quite a bit.
We're having fun. Ignoring a lot of the rules. Keeping it simple. He mostly wants to drive around and look for (or cause) trouble. He's had run-ins with bikers, drug dealers, mafia types, etc. No monsters, paranormal stuff, aliens, or any of that. Just "real" world situations. And lots of shopping.
He's having a great time, and he's also been more attentive during D&D games, so I consider this a win-win!
[JB -- I did look briefly at the sneak peak of CDF you sent me to consider as an alternative to d20M, but I've been too busy to give it a proper read through. I'll get to it, I promise!]