Natural light is more pleasing than harsh flashes. Be aware of lighting. Late afternoon is the best time to shoot, because the light is warm and produces nice textures. Morning works, too. Avoid noon lighting, however.
Overcast days are also good, since they produce even lighting.
Don't be put off by bad weather. You may get a great portrait of your kid in the pouring rain. Stand with your back to the sun, but make sure your subject isn't squinting. Remember, silhouettes are portraits, too. If you're at a beach or lake, have your kids play near the water while you shoot into the sun.