A smart photographer comes to a photo shoot prepared to entertain and bring out the best in his subjects. Simply saying "smile" often means the mouth is in the right position, but the eyes don't have the right twinkle. Try asking family members to close their eyes, then open them on a count of three to avoid having one person in the shot blinking when the toddlers are smiling sweetly.
Don't be afraid to joke around and have fun. It never hurts to ask your subjects for one serious shot and one silly one. Once the formal shot is complete, ask everyone to stay in place and relax for a few minutes. Or plan ahead for an inside joke: One family with teenagers found rose-colored glasses for their parents to wear, while the mischievous younger members sported dark wayfarers and posed with an attitude.
Sometimes, the best shots will show up in the outtakes, when everyone is relaxed and being themselves, and not worrying about putting on their best smile!