Here are a few ways children can add more exercise to their lives:

Joining any type of local club or league that involves being active.  If possible, get a friend of the child that is interested to join as well in order to encourage participation.

Replacing regular video games with those that require physical movement (Dance Dance Revolution, Wii Fit, Wii Sports, etc.)

Spend more time playing with friends outside instead of watching TV or playing regular video games.

While watching TV do some type of exercise (situps, pushups, jumping jacks, etc.) for the duration of commercial breaks.

Encourage an older sibling, parent, or caretaker of the child to exercise with him or her, even if it is only a 10-15 minute walk every day.