I agree with the above, get "Wii Play". It comes with an extra wiimote and it is only $10 more than just buying the remote separately.
You might want to look into getting the silicone skin for the Wii Fit. Ours is getting a little dirty.
If your son is into sports games, Tiger Woods golf for Wii is a blast. You do all the swinging, putting, etc with the wii more and it is much more in depth than Wii golf. Also we have Brunswick Pro Bowling with is also much more in depth that Wii Bowling.
Carnival Games might be a fun one for your daughter. It is really just a collection of lots of little games, like Milk Bottle knock down, ring toss, games like that. Game Party is also another good one which is really cheap and has darts, shuffleboard, air hockey, basketball...
We have some kind of Beach Games too, but I think that one is the least fun.
Lynx's Crossbow training is pretty fun too, even though it is just a tutorial that comes with the crossbox gun.
It is funny though, I have gotten so much mileage out of Wii Sports.