Hello Customer, welcome to Just Answer.
For the list of game supporting "motion plus", you can look at this link
. Some game do nothing more with motion plus, some use it when a motion plus controller is present and some absolutely require motion plus.
It's hard to suggest game not knowing the genre they like the best and how skilled they are (for example if they finished F-Zero and Viewtiful Joe at 100% on game cube they are really skilled :)). The $199 25th anniversary red Wii come bundled with 2 games, Wii Sport game and Super Mario Bros Wii. None of them use the motion plus. To get an extra motion plus controller for cheap, you can look for the sequel to Wii Sport (Wii Sport Resort) as it's bundled with a motion plus controller and use it fully. Super Mario Bros Wii is a blast to play and can use 4 controller at once (something to consider if they have lot of friend playing with them.
Many game use all kind of accessories like the balance board, tam tam and many more. Personally i think the standard controllers are all what is needed and i always prefer a new game instead of new accessories that are not used in much games. There is a classical Gamecube controller for Wii to play Gamecube game on it, but as they already have the real thing it make no sense getting this.
For the reviews, a good resource is http://www.gamerankings.com/wii/index.html . You can complement this by checking video on Gametrailers.com (they also have detailed video reviews).
Lastly, the important advice. I also work in the TV category on this site and take my word for it as i saw that more than you think: The priority is to have them understand that the Wiimote can easily fly right in the TV screen and break it. Use special parental authority to make sure they (AND THE FRIENDS), put the security strap of the Wiimote at all time.
If you tell me the kind of game they like i could suggest some title. Right away, you can't go wrong with the 2 Mario Galaxy games. The Sega all star racing game is very good and so is sonic colors. Kirby's Epic Yarn if very imaginative. Donkey Kong Country Returns is a good. Super Smash Bros Brawl is still the best fighting game on the system.
Edited by Martin on 11/30/2010 at 9:06 AM EST