For a game they could play together? I think it would be worth it to check out "Legos Star Wars: The Complete Saga".
It's our 4 year old grandson's absolute favorite and he plays it either with my husband or our 15 year old nephew--it's his favorite game too. It's great for kids of all ages! And they play the game working together as a team--not in competition with each other. Solving puzzles together while one works on it, the other watches his back. With 36 different levels featuring the best parts from all 6 movies, it offers plenty of variety and challenge for just about everyone.
I'd also highly recommend getting "Super Mario Galaxy". As possibly the best Wii game made to date, it belongs in every family's library. It can be a little disorienting though, and as it requires good coordination, may be a little too challenging and frustrating for your 5 year old--at least right now.
For your older boy, I would recommend "Zelda Twilight Princess". An action/adventure of epic proportions, your biggest problem may be getting him to curtail his play time.
If he likes music? Guitar Hero 3: Legends of Rock might be a good choice.
As far as games for your 5 year old? Until we found "Lego Star Wars" we had a problem finding really suitable games our 4 year old grandson could really play and enjoy. Most are a little too challenging and frustrating for his age level.
A really good "first party shooter" game for him might be "Elebits", which not only our grandson enjoyed, but enthralled my husband to the point where he spent a couple weeks solid playing it until he progressed through all 27 levels! Although it's primarily a "shooting" game, your weapon is actually a "capture gun" which you use just to trap the "Elebits" to use them to power up electronics.
(Cosmic Family? Well, at the age of 4, our grandson just finds it too simplistic--he got bored with it after one hour, but I think for younger toddlers it might be great.)