As noted by sashbar, ISO speed may be your limiting issue with the camera. Also, as a full frame Canon, your EF-S lenses, if any, won't mount to the camera.
Low light shooting of moving subjects (even posed!) requires high ISO speeds, fast glass, and shutter speeds generally 1/125 and faster. The exposure triangle tradeoffs can sometimes be brutal. When I had my 60D, I could sometimes 'push it' to ISO 3200 and get acceptable results, but most of the time, I preferred 1600 or 2400 for low light work. That sometimes resulted in a bit too-slow shutter speeds to freeze motion. 1/20th, 1/40th, etc, you'll get subject motion blur in 4 out of 5 shots...or worse. And as preschool kids aren't known for sitting still longer than 1/125th of a second, you'll likely need fast glass and be forced to deal with DOF too narrow for group shots larger than perhaps 6 kids.
In short, if you're looking to buy used, I think I'd go with a used 60D, 70D, or even a 7D. That way, you get a camera with decent ISO capabilities, perhaps better auto-focus abilities (I never used a 5D or 5Dii), and if you have EF-S lenses, you won't be forced to upgrade unless you need something in the f2.0-2.8 range. Also, in smaller spaces with a crop sensor, a group of 5-6 kids 10 feet away will require a focal length of 16mm, give or take. The good news about 16-30mm or so on a crop body, is there's very little keystoning (converging/diverging vertical lines) as the distortion is more apparent closer to the outer areas of the image, which is cropped away by the camera.
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