Its kind of funny that the Wii is often labelled as a 'kiddys console' yet there are lots of people here saying the 360 or PS3 would be better for 5 and 8 year olds.
No Skate on Wii. There are a couple of Tony Hawk games, i think they use controls similar to the other games except for waving the Wiimote to do tricks.
War games, Wii has Call of Duty and Medal of Honor: Vanguard. Medal of Honor: Heroes 2 comes out soon with an onrails option so you just point and shoot with the Wiimote, so it should be easier for little kids. Battalion Wars 2 looks like a suitable war game for kids too.
Godzilla: Unleashed comes out on Wii in November.
For Star Wars games the Wii is getting Lego Star Wars which is basically the 2 previous games combined and will let you wave the Wiimote for characters with lightsabers. There were also 5 Star Wars games on the Gamecube that can be played on Wii (including the excellent Rogue Squadron games).
There are lots of other kid friendly games on Wii as well, Mario games etc.
Im not sure about the range of games suitable for 5 and 8 year olds on 360 as the only 360 games that i really know of (other than multiplatform games) are Gears of War, Bioshock and Halo.
I wouldnt recommend the PS3 at all for 5 or 8 year olds, except for that it plays PS2 games. It seems more marketed towards multimedia users and mature gamers than kids.
If you are willing to spend PS3 kind of money though you could almost buy both a Wii and a 360 and have the best of both worlds.