I too agree that you have to set the example, and set the rules. It is a child's job to explore his/her limits, in everything. They explore behaviors and try to exert their will over the will of their caretakers. They will try bad behaviors as well as good ones. They don't understand the concepts yet between the two, at least at the pre-school ages. But they do understand yes and no. They are also beginning to get a feel for what works and doesn't work for them.
It is the parents job to help their children grown and learn, and to teach them values that they will need through life, such as proper nutrition. They will push at you, try to get their own way. You need to push back, to protect them from doing things or developing habits that will hurt them. They will respond to love, and to consequences. When they do things that help them or the family, the consequences are increased positive attention, and maybe, once in a while, a treat. Mostly, it's letting them know you are pleased with them. When they behave badly, that attention they crave should be removed, and negative consequences need to be applied, such as time outs, time alone in a secluded space, and not giving them the thing they want. Your children are your most important responsibility. And you are the greatest influence in their lives. So it's up to you to be the parent. Give them boundaries. Don't let them rule you. You and they will be happier and healthier. The bad behavior usually goes quickly away. They may try some new behavior that is wrong, but again, it will soon pass if they find it isn't giving them the desired results. Reward them only for good behavior, even if it means eating those steamed peas.
Seeeeeeeya; Goodweed of the North