Having just hosted a sleepover for 10 and 11 year olds last month, I will suggest having all batteries new/recharged.
If you can borrow extra controllers/numchucks, it lets more kids play at once. Two sets are okay, though. You have to remind them to take turns.
Let them set up their own Mii's. That'll take up at least 45 minutes of time, hahaha.
I second the Cooking Mama and Mario Party game suggestions. I even like those games.
I've rented/borrowed games and equipment (like the Wii board) to try them out before we buy. It's a good way to test things out.
Also, look up "cheats" for the games you already have; unlocking secret options makes the games new again.
I wouldn't recommend Guitar Hero - most of the lyrics and graphics are not for younger kids. You wouldn't want to let someone else's kid play that game.
Rock Band is a great alternative and a lot of fun for a group. If you have a guitar, it works with this game too. If you don't, it's okay - they are other instruments being simulated with the numchucks and controllers.
If you have Wii Fit, that's always a hit, but it's sometimes hard to find in the stores and it's a bit pricey.
The dance games, like Dance Dance Revolution (DDR) are good, but you have to buy the dance board, so it's a bigger investment than just a game.
Good luck with the sleepover - been there, done that, survived the trip. (Just last month) Word of warning: if you DON'T want them doing beauty makeovers, make it clear to each and every invitee. I had told my daughters "no" before invitations went out but
technically, I didn't say it to the guests. One of the kids brought nail polish so they could do manicures and pedicures after I went to bed.

I overheard the plotting and resolved it before they ruined my hardwood floors.
I was fuming and my kids got some lecture the next day after everyone left. I don't think that girl's ever being invited back because I just felt that was disrespectful to me, not to mention sneaky. That was the third strike - last year, she started a popcorn-throwing fight. This year, she also apparently threw scrambled eggs at someone, which we found on the windowsill while cleaning up after the guests left.