Get on the floor in plank position and let your little one (or little ones) sit or lie on your back. According to Chadwell: "They usually don't hold still, so your planks become quite the challenge."
If your kids don't quite weigh enough to challenge you in a regular plank, step your game up and do walking planks. This will give you variation and give your kids a real ride on your back. Even if you feel you're not getting long planks or enough weight, don't worry because you'll be working on your stability more than any other plank just by balancing your kids!