No matter how much time it takes you to set up lights or scout out the perfect location, by the time your subject arrives they may have a different idea of how long you'll have to photograph them (especially if they're kids.) The biggest mistake you can make is trying to pressure them to get that perfect smile or the ideal pose. It's better just to embrace the situation! Work fast with a few fun props and move around to get the most variety. Sometimes, MOST of the time, it's about capturing someones true personality and not that "perfect smile".
Here's a 10 minute photo shoot with Portrait Independence and a little boy about to turn 3, ending in some casual, spurt of the moment, musical performance, head-shots for his mom.