Umm....The answer about the first question its easy...you can use WWg models to play anything you want. About the games....D&D (medieval fantasy) its a good game to start. With the 4th edition release i think you could find the rules 3th edition rules books cheaps.I like Anima too, but its dificult learn the mechanic for new players.
Sci-fi may be Heavy Gear (Corporations in war, mech, space travels), Star wars or my favorite...Cyberpunk. All are very easy and funny to play.
Really you can use WWG sets with any game...you just must find the ideal genere to run your aventures. If the teens are never play a RPG. you should look for a game with an easy system. This with a good history plot, some miniatures and WWG sets can be a great and funny experience to the childrens.I hope my answer be a little usefull. Good luck!!!!
