I agree with the idea of engaging with your kids. Our youth are some of the most active humans on the planet and if anything, they have a great grasp on staying active and more importantly, having FUN doing it.
Walking with your children is a great idea, it will allow you to relax and enjoys their company. As an adolensent nature is something interesting that can be explored while learning and having fun. This will excite them each time you want to go out for a walk. Not long ago my mother and I went to Yosemite National park in California, the constant movement on foot was great, but we really had a blast climbing a bunch of rocks at one of the falls inside of the park. I could tell she was having fun and interested, but she didn't exhaust herself either.
Another awesome method is simply allowing your kids to mess with you while you work out. Sometimes the stress of being disturbed or challenged puts an extra charge in your system to really get the most out of your workout. Have them push you down every time you do a push up or do push ups with you. Compete to see who can get the best results, whatever that may be(most push-ups, who does the most fundamentally correct, who can do a push up hold the longest, etc.). The point is, kids can be the easiest, cheapest way to stay active and have fun doing so. Activity=Fitness.
Also, if this happens to be not as convenient to you, there are other options to getting the results you want here, on the web. I am subscribed to an AMAZING newsletter. The writer constantly sends Emails for free give aways and tools. The letter constantly updates methods for weightless and muscle building/generation/restoration. I wish I could post the link here, but it's in my profile. Even initially, for joining the newsletter, I got a FREE report. I was astonished how much value I was getting right from the start. And for free! The guy does do some products that are also very helpful, and for those that can afford it, they're REALLY helpful and worth it. However, he's not a spammer and really gives great advise on this stuff. I recommend you join the newsletter, the link is on my profile, or you can PM me, if you want to chat.
Hope I helped!