another question, would getting a speedlight (430ex or 580ex) or ST-E2 helped me with a more accurate focus in this situation? Or those will only helps me in low light condition?
thanks.
Only in low light.
You need to practice your AF skills more. Once you realise what you are doing wrong it's easier to get right. You will not get every shot right but choosing the best aperture, shutter speed & ISO for any given situation will help a lot!
Shooting at f1.8 from 7' with a baby that is moving back and fro is unlikely to be the best settings..... Try f8 and use a flash not for focus but to add light to the scene to allow you to get f8.




