Too often when we take pictures of other people, particularly parents taking pictures of their children, we insist that they look straight at the camera and smile. We parents will often give the instruction to, "Say cheese!" or some other such silly phrase. While this may elicit a smile for your picture, the smile is rarely genuine, and you can usually tell that in the photo (especially if you're the parent or other close relative).
My advice is: forget about the smile! That's not to say you shouldn't try to take some pictures of your kids smiling, but seek to capture genuine smiles in candid moments. They'll be much more genuine, and that true, happy smile will make you smile when you look at it later. Also, realize the value in capturing other emotions, moments that don't include a smile. The picture above, for example, is of my daughter, Marisa, and she's not even looking at the camera. She's looking at something off to the side and away from the camera, but it's a great moment captured, and one of my favorite pictures of her.
In the picture above, I have her looking at the camera, but you can tell she's not thrilled about it. I love this picture, though, as I love how it makes you wonder what's going on it that little brain. Again, a smile isn't always necessary...
This last picture was also taken that day (these photos were all taken at Clinton Gulch Reservoir, on Fremont Pass, by the way). Again, here she was focused on something else entirely; didn't even realize I was there taking pictures. I love the expression and the totally candid nature of it.
So the next time you're out taking pictures of your kids, try going beyond the "cheese!" shots, and just capture them as they are. You might find they end up being some of your best photos. And if you hire me to take portraits of your family, I like to work in a candid manner this way, too. We'll take some posed shots, to be sure, but I also like to capture some of the family just relaxing and having fun, experiencing real emotion. Kids at play are the best. Naturally, location shoots (your home, a park, etc.) are more conducive to this than studio shoots, so it all depends on what you envision, where we shoot, etc.
Now get out your own camera and go have some fun! :)