Think your child is too small to exercise with you? I walk or run after my 4 y/o as he rides his bike ahead of me. I have him stop at every intersection so I can catch up. It keeps my pace up, helps me to keep an eye on him, and encourages family exercise. He is always shouting, "Run Mom!" when I slow down, so it's like having a free personal trainer!
We used to walk together, but he was so slow I was constantly having to stop and wait for him, or slow my pace so much that my calorie burn was not high enough. Now I get real cardio every morning and afternoon, just by being a good mom!