Originally Posted by
DanK
What do you want the lens to do for you? What should it do that your current lenses don't do? Unless you specify that, it will be hard to give you good advice.
Personally, I would almost never want something shorter than 40mm for candids of kids. In addition to the issue that Allan raised--having to get very close--wide angle lenses don't provide a very flattering perspective. Telephotos flatten because they increase the working distance; wide angles do the reverse, leading to bulbous noses, etc. One can use any length for portraits, but the conventional portrait range for FF cameras is perhaps 70-105mm. I just looked at a few of my recent candids of toddlers and young kids, and they were all in the range of 80 to 105mm.
Consistent with Kathy's suggestions, my standard candid setup for kids indoors is a 24-105 with bounced flash and a bounce card (Demb Flip-It). It has precisely the focal length range that I most often want. There are several 24-70 lenses that are optically superior, but given that I often shoot kids at more than 70mm, this works fine for me.