I can't relate to photographing children, but I do photograph live local concerts in tight spaces in near darkness. Considering the performers move quickly, I pair my 5D mk ll with a 430ex ll and am able to get wonderfully clear shots. Personally, I would buy an external flash with your purchase because it really does open new possibilities and unlike the built-in flash of Canons lower models, their line up of external flashes don't make the image look flat or dull.
As far as AF goes on the 5D mk ll, I have never found it disappointing. Sure it isn't a system that would be ideal for sports, but for general use it is the first body out of my bag. With your listed subjects in mind, I believe you will be pleased with your purchase.