We love our Wii! I have a five year old boy and a almost-four year old girl. We do have the Wii Sport which is great for this age range and for adults (that may/may not be into gaming) too. You do not need the tennis-golf club kit... you can definietly do the games without those attachments. It is a good idea to get another remote and the nunchucks - I would highly suggest that, especially with four kids and two adults. The nunchucks will be used in the boxing and if you purchase other games, you will probably need that too.
I do want to suggest some games since I wish I had known better...
-Wii Fit is also good for the age range that we have at home. Get it in the combo pack to include the board.
-Wii Resort (get that in the combo pack too).
These will get them off their behinds and they will actually "play".
Good luck and Merry Christmas!