I haven't read through the entire thread yet since I'm short on time right now and will edit this post later, but what I'd do having skimmed thusfar;
* Used 60D at around R6.5k - if you're lucky you'll pick one up with a grip and extra battery for R7k or so.
* Used Sigma 30mm f/1.4 prime at around R4k (not a 50mm, since it's a crop camera and 50mm can more often than not be too tight for practical indoor usage)
* Flash cable at around R150, seeing as you have a flash cable or using the 60D's flash commander functionality to control your flash. Depending on the settings you're using, the popup flash may not show up in the resulting photo all that much at all, and so mightn't present undesirable results. Otherwise, make something to diffuse it with or simply use the remote cable.
You can consider getting electronic extension tubes for macro usage at this point. If I recall, you should be able to achieve a roughly 1:1 reproduction ratio on average using a 28mm tube on a 30mm lens, and retain AF and aperture control with this method while not exposing the rear element of your lens. A Canon one will set you back R1k while a Kenko automatic tube set will be around the same and include three different sizes (that can be stacked).
Also,
Sigma lenses are crop-only if they have 'DC' in their name. If they don't, they're compatible with crop and full-frame.
The bigger concern with using crop-only lenses on full-frame bodies tends to be the mirror hitting the rear element. On some crop-only lenses this won't happen, you'll just begin seeing the inside of the barrel instead. An example of a FF-capable lens is the Tokina 11-16mm ATX etc etc f/2.8 lenses, which can be fitted to a full-frame body, but by the time you zoom down to 12mm you're seeing a lot of the inside of the barrel.
*edit* Okay, having read through now, it appears you guys need to first assess what it is that you want out of your new equipment and to understand what you'd be getting for what you'd have to pay.
I recently handled a 1100D, and noticed immediately that in terms of its controls, it's drastically more frustrating to use from a 'semi-manual' viewpoint than just using it on one of the auto modes. The 'ISO' button near the shutter by default controls the popup flash, as there's no dedicated flash button aside from that one. One can set that button to act as the ISO button instead, but then you have to put your flash popup somewhere else.
On the friend's 1100D I was handling, I did manage to set up a control layout that allows for more efficient control of the body in general, but it's still significantly more cumbersome to use than any triple-digit body and significantly more annoying to use than a double-digit or greater body.
If your requirements primarily lie in getting better photos of your baby, I'd go with the suggestion of getting some remote triggers and using the flash off-camera. Your baby will eventually get so used to it that they'll completely ignore it, and if not it could make for some interesting photos if they want to play with the flash instead. At least until they get bored of the flash and finally ignore it.
For macro use, I now understand you actually have that Sigma 70mm macro lens already, so you don't need extension tubes, just a way to better light your subject. The 60D and 7D can control a 430 EX II using the popup flash as I mentioned above, while the 6D can't since it has no popup flash.
If push comes to shove, a 6D would definitely be a great upgrade for her in terms of the high-iso usage. I still don't get why it is, but
all Canon's EF lenses seem to also perform better on full-frame bodies, too, so even if you get a potato like the 28-135mm for a 6D, it's
still going to (arguably) perform comparably to, say, a 15-85mm would on the 60D or 7D. This while being a relatively cheap lens you can pick up for around R2.3k or so used as a stopgap until she can warrant getting a 24-105mm.
Your Tamron ultrawide zoom (which is crop-only - the P&G guys dun goofed) can be sold used for around R4k if it's in good condition, which will cover replacing it with a Canon 17-40mm f/4L, which with a filter offers you a weather sealed-platform of the 17-40mm + 6D. Otherwise you could use the money towards a 24-105mm f/4L or at least to get a 28-135mm f/3.5-5.6 IS USM and 50mm f/1.8 for the 6D.
Then at some point she can replace the Sigma telezoom with, say, a Canon 70-200mm f/4L IS USM (the expensoov one), which is again weather-sealed, and eventually the 24-105mm f/4L IS USM, which is again weather sealed. Good f/4 constant aperture kit that'll last her very long, unless something really magical happens in the photographic equipment world that means you
absolutely HAVE to go out and buy those new goodies for her.
So tl;dr, at this point I'd almost say to cave and go for a 6D and a 28-135mm. As bwana's pointed out, being able to use a higher ISO is often more important than being 'forced to' use a wider aperture, since you may find yourself at an event where you want to get more people in focus, but can't use a flash (or it simply wouldn't be practical/make enough of a difference), and could then push the ISO to make up for the lack of light. Based on the testing I've done thusfar, a 6D can easily provide as good as, if not better, noise performance than a 550D/60D/7D shooting at ISO800 when shooting at ISO12800. That's 800-1600-3200-6400-12800, so 4 stops more effective light, which with the one stop less of DoF from using full-frame means an effective 3-stop improvement.
Keeping this in mind, consider that on the crop body, for roughly equivalent noise performance you would be using a f/1.4 aperture, whether a 24, 28, 35, 50 or w/e mm lens.
The kicker is that on the 6D, you can use a 50mm f/1.8, which as you probably know by now, is a relatively inexpensive lens, and it'll provide you with the equivalent field of view that a ~30mm lens would on a Canon crop body - so if you really need to use a higher aperture lens, at least you have a wider field of view to work with without having to spend the R5k or so that a Sigma 30mm f/1.4 costs.
You're also not then restricted to your crop-body 50mm equivalent lens for low-light shooting, since your body's higher ISO capability can make up for the loss of aperture, allowing you to use the entire 28 (17.5mm) to 135 (84.5mm) range the lens offers as a zoom lens.
Oh, and if you end up deciding to simply go for a triple-digit body, I might be willing to part with my 500D+grip+2x batteries for R5.5k. *cough*
