I'm coming at this review as an educator second (No 13 year old kid for me, just a bunch of students), but a Pokemon player first. I've been at Pokemon since I was in sixth grade and Diamond/Pearl is the best installment in the series. In the game you play a kid on his or her journey to travel and train Pokemon in battles, but along the way you get mixed up in trying to stop Team Galactic, a cult who wants to rebuild the world from scratch to get rid of the evils of human spirit. To do this, they, sorry for the spoilers, capture and experiment upon living Pokemon. Not for the faint of heart.
The violence in this game, while not as bad as the XD/Colosseum cycle, is pretty high for a Pokemon game in what happens to several Pokemon. Also, the political controversy int his game may be too much for some smaller kids to truly grasp, especially since the translators made a few links between Team Galactic's ideology and structure and that of a controversial real world religious group, whose name I am almost afraid to mention. Aside from that though, the game takes a frank look at what kind of person would join one of these evil syndicates and how a charismatic leader can induce people to do bad things. Dark themes for young kids. Maybe your older kids can appreciate it.
Another thing I want to mention is Cynthia, the female champion of the game and a great role model. She's friendly, compassionate, generous, always searching for the truth, and extremely tough. As a feminist I was glad to see her included, but also the fact that she's a woman champion almost completely unmentioned by the game. If you play on the level I do she doesn't seem so great given the implications of how high her Pokemon's stats are and how she must have rejected tons of other Pokemon to get such good ones, but most players will not reach that conclusion.
I love this game and I still play it regularly. It's my favorite Pokemon game and I recommend it to the oldest of my younger cousins and even my teenage students, but sometimes I worry about what my six year old cousin would make of Team Galactic and Cyrus.