I don't have much experience with playing games with kids. I do however read up about different games through online & magazines, such as PC Gamer & Maximum PC.
I would suggest PC games such as, Lego LOTR, Back To The Future, The SIMS, Peggle (very fun), and Lego Star Wars.
On the console side, anything Mario related are always fun & popular, Angry Birds, Sonic related games, maybe Rayman too.
Board games, Stratego, Uno, Jenga, Clue and for the smarter (maybe older bunch), RISK (any variety) 😀