Our customers have some great questions and though many people think their question is "silly" or whatnot, they're not the first person to ask it and they definitely won't be the last.
We recently had a customer who wasn't sure if her focusing issues were operator error or if there was an issue with her gear. The solution came from something that we see fairly often: Using continuous shooting mode and/or focus tracking (AF-C or AI Servo) when shooting relatively stationary subjects. Now we will totally admit that children/pets/wildlife are hard to control and they're more likely to be in motion than still which makes continuous shooting very appealing. The trouble can come when shooting portraits in this mode - with modes that track (ie. the camera continually adjusts the focus depending on where the focus point is) there is no ability to lock on to your subject's eye. The result is that any slight movement of the focus point away from the eye (or whatever you are trying to focus on) will result in a shift of focus leaving an ear sharper than an eye, etc. In portrait work, the photographer is usually closer to the subject which effectively decreases the depth of field of a lens so having the focus point just-so is far more critical than shooting wildlife that is many yards away (further distance from the subject increases depth of field) so there is generally more wiggle room there.
The moral of our story: When you are photographing children or pets from a closer distance, don't use a continuous mode (which usually automatically enables focus tracking) unless you feel very confident in your ability to keep your focus point exactly where you want it. Instead, shoot in single shot mode and increase your shutter speed to make sure that you freeze your subject, lock your focus on your subject's eye, and practice a bit until your shutter release timing is pretty quick.
We hope you found this tip helpful! Feel free to call or come by any time and ask us any questions that you might have!