You mean ISO 6400 and f2.8 wouldn't give you enough shutter speed to stop your child in her tracks in ambient light? Heck, I manage to stop galloping and jumping horses with ISO 3200 and 6400... So your kid must be sheer lightning.
5DII can be used at 6400 without much concern... Even 12800 can be surprisingly good. Just avoid underexposure to keep noise to a minimum. If shooting JPEGs, you might need to increase noise reduction in the camera. If shooting RAW, even noisy high ISO pictures might be usable, if post processed right. Or converted to B&W.
Biggest "problem" with 5DII is its slower AI Servo tracking... Can't keep up with a lot of moving subjects, particularly in more challenging lighting situations. I have more missed focus issues, than exposure problems. 7D work much better tracking fast moving subjects.
If lighting is low, you will have trouble manual focusing too. That's a steep price to pay for 1 additional stop of light through the lens. Zeiss glass is pretty fabulous (even if it's now owned and made by Sony).
Are you using Tv, Av, or P with your flash? If so.... assuming it's a Canon flash.... you are only getting fill flash (unless you set it up manually). Switch to M and set the shutter to 1/200 (camera might default to that)... Then you will get full flash.
Fill flash is where the camera is still using auto exposure mode to adjust for ambient light as the primary light source, then just using a little bit of flash to fill the shadows as a secondary light source.
Full flash is when the camera is using the flash as the primary light source, no matter what the ambient light is.
Plus, if it's a Canon flash you can set it up to give Focus Assist, which will help the camera achieve and maintain focus.
Your 24-70 has 35mm focal length as part of its range, too.... Not that the Canon 35/1.4 isn't a very nice lens! It certainly is. Without knowing your level of experience ("amateurs" and "hobbyists" can be more experienced and knowledgeable than "pros"), just judging from this brief post, I just can't help but wonder if you might need to learn more about how to use what you've already got.