What is considered low light?
Any condition that is likely to push the camera/lens combo to its limits.
Most indoor daylight conditions would be considered "low light" even night time with "normal" electric lights can be somewhat low.
Also, what do u mean by shoot in the dark? Sorry just need a reference point so I can determine myself if the 15-85 is ok for me.
If you are shooting indoors with the 15-85mm, you will want a flash. I wouldn't recommend this lens for photographing your growing son -- it's just not the best fit for this. Having a fixed f/2.8 aperture is more useful (higher shutter speeds indoors, more background blur)
Bottom line, I'm getting the 60D through the loyalty program. I have $1500 to spend and the body alone is $640. If u were me where 75% of your pictures will be your child, what would you buy with the rest of the money?
A flash and fast lenses. I bought my first SLR under similar circumstances to you (it was a Rebel XS for $400) and then got the Canon 50mm f/1.4, a flash and the 15-85mm. I ended up selling the 15-85mm, but the 50mm f/1.4 became my favorite lens.
Given what you wrote above, I'd recommend something like the Tamron non-VC (get this used), the 50mm f/1.4, and a decent flash (e.g. 430EX). That would get you close to 1500.