Kids love copying adults.
To quote Sean at our Junior Kids class 'my daddy is a strong man because he trains every day, but I'm going to be stronger' (sorry Darren ).
Your everyday actions, whether they say it or not, are noted by any kids or young people in your life. This isn't a guilt trip, but more to remind you how much influence you have on the actions of the next generation.
If you make exercise a part of your everyday life, the chances of your child doing so are higher, as numerous research studies have demonstrated.
Which increases the chances of your child leading a healthy lifestyle - along with all the other benefits of exercise... Sounds like a win-win for everyone