Here's an interesting, computer-animated video (no voice over--text only) from The History Channel on samurai. This one goes a little more in depth on their weaponry--again, text only--with a little live action, and pretty interesting. Your boys may eat this up. Note: Does have a dramatized, History-Channel limited-blood demonstration of seppuku (Japanese ritual suicide).
These photos from National Geographic give some great images of samurais as well as these.
Here's an overview for kids of Yoritomo and the Japanese feudal system.
For more interest, in this video clip, a host from the History Channel is dressed in a samurai suit, and participates in medieval combat with a female samurai. In the end, his loss means he has to pay honor to the female samurai.
For those of you with older students, The History Channel has a lot of articles on shoguns this week--including an article on Minamato Yoritomo (see images here), as does biography.com. There is one History Channel video on seppuku, but I don't think I'm going to watch it. :)