If I had kids I don't think I could afford losing the energy boost exercise gives me, for their sake.
Not to mention the huge benefit of kids seeing their parents exercise. My 6 year old asked to go for a "run" with me last night, so we ran a ~500m loop together before I went off on my own. We both really enjoyed it :)
It's a bit tougher with kids but not much tbh (obviously it's harder if you're a single parent). You just have to do it in the morning before they get up, after they go to bed, or during lunch hour.
If you are chasing after your kids you are doing more than enough exercise.
I can't tell if you're joking or not but this is not close to being true. See for example the millions of obese parents in North America.
edit: to expand, it's unlikely you're getting your heart rate elevated to see much improvement in your cardiovascular system or doing much of any resistance training.
Thats because those millions of obese parents mostly sit on their couch, play video games and drive kids around. Start actually chasing them.