I can focus and take photos of my 2 year old indoors just fine with my f/2.8 lens. With the 5D2 you can dial up the ISO and not use aftermarket noiseware. If it's still too dark you can always add a flash which will provide light (duh) and focus assistance if it's a newer Canon flash.
Note that the AF assist light only comes on in one-shot, and won't in servo. I dunno why, but it vexed the heck out of me until I figured that out. Since I generally am in servo to track my active toddler, mine rarely sees use.