Spend time doing things with them. Children learn by example. Showing them how to be proactive will end up with them doing the same. Spending time with your children, will lead to them wanting to do more. Taking them out for activities, supporting them in sports and such. Naturally, people want to do things. Its being lazy that gets us all. Spending more time on the couch, will only teach our children that its right to sit around on our bums all day.
Also restricting them will only lead to them not wanting to be active. Taking them out and let them run and play, will let them enjoy themselves, and want to do it more.