Children love to have fun. It's why they love computer and video games. They're fun. It's why they like to watch television shows and movies.
If you mention the word "exercise" to them, all thoughts of fun disappear from their minds. Exercise is push ups and jumping jacks. Exercise is endlessly jogging around an oval track. In other words, it's boring.
It doesn't have to be. It wasn't when we were kids. Of course we didn't have all the cool gadgets kids have now so we had to find something to do that was fun. Most of our outdoor games were very vigorous in nature. They still are, they just aren't played as often.
Making it fun : Take the bikes to the park and let the kids ride them on the paths. As long as they are polite to walkers those paths are just as useful and the kids are infinitely safer than riding on the streets.
Find an organized sports team that the children are interested in. Our area offers baseball, softball and water polo. The schools add volleyball, basketball and football. This will do more than provide exercise, it will teach team spirit and fair play.
The memories I enjoy the most when it comes to this topic is when adult family members joined in the games. My parents would sometimes play and older cousins as well. I can still hear my mother yelling "Don't you ring my tomatoes!" when we'd get into a horseshoe match. (The pit was right next to that part of the garden.)
Getting involved helps bring the family closer together. It teaches that adults like to have fun and that this sort of thing is life long. It's well worth the time.