Typically between eight and 15 months, a new event or activity will cause a child's attention to shift. This is instinctual. If your child has trouble sitting through a book reading, read a favorite he can sit through. If you finish that one and you feel he can handle another, read a second favorite right after the first. Don't pause for too long or you may lose him. Keep increasing the length of reading time in each session, but let your child be done when he's done.
Between 16 and 19 months, a child can sustain attention for two to three minutes. This is a great time to experiment with secondary stimulation. If she is putting a puzzle together or playing with a toy, put music on in the background. This forces her brain to listen and interpret the notes in the music while also focusing on the task at hand.
Between 20 and 24 months, a child can sustain an activity for three to six minutes with or without an adult. A game of rolling a ball back and forth can hold his attention for a while. When he begins to focus on something new, take out another ball and roll him both. This new development will refocus his attention as if a new game is being played. Develop games further to hold his attention for longer periods of time.
Between 25 and 36 months, a child can focus on new things then refocus on what she was doing. One example is listening to an adult then going back to the puzzle she was putting together. Use this ability to refocus by prompting her attention when it shifts. This is the time to teach her the importance of paying attention. Don't attempt a complicated explanation with far-reaching concepts; only explain that it is important to learn. While in the grocery store surrounded by stimulation, recite the grocery list and ask her to repeat it back to you.
Between three and four years, a child's attention span can last up to 10 minutes and can alternate between activities. Play one game for as long as he can, start a new game, then go back to the first game. This will help develop his ability to multi-task and improve his memory while developing his attention span.