1. Children Being Children
Obviously you need models. Ideally, you will allow your young models to go about their business. That way you get the most natural behavior, the happiest smiles, and great action shots.
When my friend lamented how hard it was to get a picture of her four month old wearing a Rainbow Hat for me, I encouraged her to just let her daughter interact with the hat naturally. Let her pull it from her head, play a game of "drop" with it, or otherwise just be herself with it. Those are the pictures I was after...not the studio shot.
2. Fast Camera
It is a HUGE BUMMER when you miss the moment because the button on your phone's camera is glitchy or your point-and-shoot is slow. I am eternally grateful for my Nikon Digital SLR. (A Christmas gift ten years ago! It is old but it still gets great pictures for me.)
3. Sense of Humor/Fun-Loving
I find it helps to have a light heart and sense of humor. If I interrupt my kids when they're enjoying themselves, pull and Rainbow Hat on their head, and hurry them out the door to get the shot while the light is good, then things don't work out.
I have more success when I say, "It looks perfect outside for a photoshoot. Will you help me out today?" We have an abundance of time, living a slow life, so I am often answered with, "Sure!"
I also try to choose a place with a neutral background. Wilderness spaces are best but anywhere with trees, grass, and sky works. Then I use the trick of standing far away and zooming in to create a blurry background. So I stay strolling behind my kids and let them go be goofy, have fun, stop and smell the flowers....whatever moves them will likely make the photo great.
That's how I got this picture... ("Sure you can bring your Wonder Woman doll. Oh! Smelling the flower is a great idea!" etc)
Speaking of fun, I took a balloon to a photoshoot and (with the help of parents who played along) I got some giggles and other great shots. The parents stood out of the frame playing and being silly with the balloon and my model just watched (and sometimes laughed) while I took pictures.
Something Else I Look For: Good Lighting
I like to shoot outdoors on overcast days. It reduces the likelihood of squinting eyes and inconvenient shadows.