When you do that, it results in a stronger, more visually appealing photograph. Of course, this is really only a guideline, there are no absolutes when in comes to art.
And now finally, the part for those who are being photographed...when you or your children aren't in the center of the photograph, its intentional. You might be surprised how sometimes you are drawn to photos like that, but now you know why. Photographs that aren't centered can be more interesting. Here are some of my examples (just imagine the same grid on these).