It's easy to get so caught up in the whirlwind of child-rearing that regular exercise gradually slips through the cracks until it disappears completely. It's certainly normal, understandable, and admirable for parents to put their loved ones' needs far in front of their own; that's what being a parent is all about. But it's important to teach kids to love and move their bodies in ways that are enjoyable to them, and that teaching process can serve to remind you of the very same thing!
Show your kids as many possible ways to be active as you can.
Remember the ways in which you had fun as a kid, and share those experiences with them. Heck, even show them the activities you didn't like as a kid; maybe you'll like them now. The options are practically limitless: sports, dance, trampolining, sledding, martial arts, hiking, jumping rope, using playground equipment, hopscotch, biking, bowling, swimming, playing catch, flying a kite, walking the dog, gardening, hula hooping, etc.
Make an outing of it.
You can plan an activity based on sightseeing or some other purpose and still build in tons of walking time (e.g., zoo/museum trips, scavenger hunts, geocaching).
They're never too young.
Work on rolling, crawling, dancing, and ultimately walking with your infants. Have dance parties with your toddler and provide her with space in which to run. Chase, swing, and toss your kiddo. Utilize toddler play structures, indoor play places, walkers, ride-on toys, and kiddie pools.
Remember that it's supposed to be fun.
Your child shouldn't feel pressured into something; he's playing, and playing should be enjoyable. Encourage anything that he shows an interest in, and help him find new ideas if or when his interest wanes.
Use the time to bond.
Not only is family bonding time a great idea in general, but your child will also likely associate specific physical activities with happy memories for a long time!