I agree that the best choice would be 17-55mm + 85mm. I have the Tamron 17-55 (far much better than the canon kit lens) and a crop sensor (400D) and use the 17-55 almost all the time. If I had the money, I would probably go for the canon version with IS, but my experience with shooting kids (with camera [:P]) is that fast lens is more important than IS. Remember that the IS does not stabilise the kid, only the camera. So if I had to choose between 2.8 w/o IS and 4 w IS, I would go for 2.8 without.


Also, I see a lot of people being extremely satisfied with the 7D, which has crop sensor. If you are not 100% sure that your next body will be a full size, I suggest 17-55. Anyway, if you choose Canon's top quality lenses, I am sure you will be satisfied whatever you end up with.