Firstly, why are you using spot metering? I would be in manual, so metering is irrelevant. If you don't have a flash meter then a couple of test shots should (in conjunction with your histogram) tell you the correct exposure.
Secondly, shutter speed is largely irrelevant, as long as it is within your camera's sync speed. Unless you have a large amount of ambient light in the studio which you would use a faster shutter speed to block it out.
As for shutter speed freezing kids' movement, also irrelevant, as the flash duration is a lot shorter than the shutter speed, so it's this which freezes movement, not the shutter speed.