I have gone through all 9 pages of replies and it seems 17-55 is the general consensus but in my opinion the "Canon EF 100mm f/2.8 L IS USM Macro Lens" is also not a bad choice either.
Since it is a macro lens it will allow you to take real close pictures of the new born and really minute details like their tiny hands/feet,eyes..etc
also 100mm focal length will give you decent working distance and you can use the same for portraits. the F stop is the same as the 17-55 (F2.8)
when the kids grow older you can still use the 100mm focal length and did I mention its a Macro lens. So it will be useful for macro photography.
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