I'm convinced that moms of little kids get their 'exercise' not from 'chasing kids around all day' as some suggest, but rather from having their heart rate accelerated several times a day!
Do any workout video or attend any class, and you'll surely hear, "Let's get that heartrate up!" But who needs that class when you have little kids who will do the same for you?!
A jump from the clubhouse. A daredevil climb over the baby gate. A reach for your hot coffee. An escapee racing towards the street. A random scream in the night. A stumble near the coffee table. A wanderer going just out of sight. First time at the skate park.
Those things get my heartrate up faster than any step class or Zumba session ever could!