Taking photos of children is fun but challenging. Keep a kids' portrait session short and entertaining. Play games with them: ask them of they can see their reflection in the front element of the lens is a good way to get some eye contact.
Fit a wide-angle lens and shoot without looking, poking the camera into their face. Get them used to the shutter sound and not having to look down the lens and smile.
Make the most of opportunities when they're still for a moment, such as when they're concentrating on a toy. Chat to them as you would with adults and once you've taken a few photos show them the results on the LCD screen, so that they feel involved.