The only Kinect title I'd recommend given your OP out of the current crop you don't have is Kinect Sports. For you perhaps Alan Wake, Lost Odyessey and some of the other more interesting exclusives would also be of interest. If you haven't already played Mass Effect on PC then that's worth a look for sure.
As for Live, I wouldn't get it unless you know for sure you are going to play online on the 360. If you're going to be mainly offline - i.e. you're sure you're going to stick with PC/PS3 for online - then although it isn't much money you won't be getting much for it either.
Live is very good for online, and with Gold you do get some other stuff - Facebook, Sky, Last FM, etc. - although TBH I don't think you should need Gold just to access online apps but I guess that's a bit off topic.
I have all 3 consoles and do pay for Live, but only because I (or my elder son) are online enough to justify it.
Your kids might like Joy Ride for Kinect, too, come to think of it. I hate it having tried it, as did my kids, but they sound older and were annoyed at the wacky controls, however some friends with young kids say they find it a blast trying to steer with their hands - in my experience "trying" is the right term but hey they're having fun I guess.
Try to be reasonable... its easier than you think...
