quick answers:
1) gauntlet: dark journey - not that great, as far as i know. pass.
2) prince of persia - i'd say thats too advanced for a 8 and 10 year old (thats 2nd and 4th grade, right?)
3) soul calibur II - excellent game, but its all about violence. magically, no blood. might be a little frustrating for younger kids, tho.
4)
viewtiful joe - good pick. its a "beat'em up" type game (which is like a classic 1980s game, where you just go thru like a superhero and beat up all the badguys). its gotta kid mode and a more mature mode (no nudity or anything, just more difficult and such). its basically a comic book come to life. very fun. easy to learn.
of the four you listed, i'd definitely (and perhaps only) recomend viewtiful joe for the 2 kids.
if you're looking for other games they'd love, that are right for the age and such, these are games i'd recomend (even above viewtiful joe).
1)
mario kart: double dash - hottest game, best choice. super fun, super addicting, easy enough for a kid, fun enough for an adult. tiny learning curve, great for multiplayer. no violence or blood. 4 players.
2)
tmnt - sorta like viewtiful joe in all regards. comic book-like. violence, but nothing awful. fun, beat'em up type game -- especially great if they like the turtles. 2 players at a time.
3)
smash bros - like tk said, great game. cartoony fighting game. 4 players. no blood. excellent graphics, fun for new and experienced gamers. plus, its a year or two old, so you could probably get it pretty cheap by now.
fzero is a fun and fast racing game. mario golf is a funny, but cool, golf game. super mario sunshine is a safe adventure game, as is rayman 3, and star fox adventures.
