So this is something I've done a number of times with kids I've worked with especially on days where the weather is too bad to go out (I live near Seattle).
Here's what you do: there are three actions- front, back, and go. You and/or your kid calls out the directions in any order and for any length of time you want. When someone says "front" you get in the plank position and either hold (until a new direction is called) or you can do pushups (I'm a pushup failure, so I stick to planks).
When "back" is called you lay on your back doing butterfly kicks or crunches.
And when someone calls "Go!" you can either do jumping jacks or run in place.
This can be an especially good if you have a bossy or strong-willed child cuz you can let them be the caller and tell them this is one time that they get to be in charge. Young kids also seem to like it a lot cuz it's good for their short attention spans. And it might not seem like a super intense workout, but I always feel it the next day, even if we only did 15 minutes.
Another way to do this, especially if your kid is not very active, is to do it between commercial breaks. I mute the commercial and we do as much as we can before the show starts again.
Oh, and another way to mix things up (if you have stairs in your home) is to add the direction "up" and then you have to run up and down the stairs till a new direction is called.
Maybe it seems like an odd activity, but it gets both kids and adults moving and is a good way to spend time together.