As non-chalantly as possible, pull out a stopwatch and say, "You have to do these wallsits."
Kid: "I don't want to do them, I'm not going to."
Parent: gazing at stopwatch "You got 10 seconds to hop on over there against the wall bud. Otherwise it's white rice for dinner for a week."
Parent: "Grumbling and pouting is a bad habit, as well as laziness. Gotta get it out of you."
You sound like somebody who has never had children.
You sound like a doormat getting punked by your kids everyday.