Ok, this is working for me now, although I find the more I learn the more I need to learn...
That's how it seems to work for me too 
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You can be sure we will help you make your wallet lighter much faster....
Are there any opinions about the pros and cons of the Sigma v Canon given the Sigma is the cheaper option? (AU$519 v AU$699 - tho I havent looked through the discount importer sites yet)
Sigma is designed for APS-C, the Canon can be used for FF as well. Personally I prefer the Canon, but others will tell you something else. The Canon is one of the most undervalued lenses in the Canon line-up, however.
No, the 35 F/2 is a very capable lens. The main reason it so much cheaper is that it is a somewhat older design, and doesn''t have USM, but one of those bee-type AF engines. The 28s are a bit better, however, as is the 24.
I think it is unless you plan on shooting portraits most of the time.
IMO, low light photography starts with F/2 and faster lenses. You may not need it for a specific shoot, but at least it is there when you need it. Generally, the faster the better, provided the lens is either as good at full aperture as stopped down 2 or 3 stops, or gets there before F/2 or F/2.2. When it really gets dark you need F/2 or F/2.2 with anything from 28 mm and up.
Not necessarily, because in order to shoot handheld you need as a rule a minimum shutter speed, roughly equal to 1/(FL*crop_factor) s. This is why you can get away with slower shutter speeds for shorter FLs. BTW, for kids you generally need faster shutter speeds, easily make it 1.5 to 2 X faster, IOW, 1/(FL*crop_factor*2)s.
I would consider those for general shooting, but not for low light in- or outdoors shooting, although the 24 probably is borderline, especially because it is very good at F/2.8 already, with a very even sharpness distribution over the entire field. But generally, they are too slow IMO, and not really much faster than the kitlens (only 2/3 f-stop).
That is not what DoF does. DoF, Depth-of-Field, only tells you at which aperture with which focal length, with which medium from what distance to what other distance, when focused at a specific point so-and-so far away, the objects in your pictures will be sharp. This means that at larger apertures (smaller numbers) you will have less DoF, and less sharp in the image, than with smaller apertures (larger numbers), although there si a limit, namely diffraction. With APS-C from F/13, altough the theoretical DoF gets larger, in fact the sharpness will decrease, because of light diffraction. With FF that is from about F/18. So you should ideally not use a smaller aperture than those.
Look here: http://dofmaster.com/dofjs.html
That's always a possibility 
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That may well be a very good thing to do . Play, and see where you find you need more speed, and faster apertures, and how much, and then select your lens.
Kind regards, Wim
