One of the biggest mistakes parents make is not recognizing when a child wants to help. Being in small bodies, they are not going to offer the same kind of help that a fully grown person does, but they will make the effort. And when that effort is not recognized AND acknowledged, then the child will start to "help" in ways you don't appreciate.
I've seen this with my own eyes: A child around two years old is sitting in a shopping cart. He takes an item from the cart and tries to place it on the conveyor belt to the cashier. The unaware mom stops the child from doing this and says, "that's okay, honey, I'll take care of that." Unfortunately, I've seen too many moms yell at their child for attempting this. Perhaps the mom is worried the child will drop the item. Perhaps the mom is in a hurry. Perhaps the mom is just stressed out.
Now we turn to the mom who sees this seemingly insignificant act as a real effort on the child's part to HELP. The child wants to contribute in some way. She's small, but she can grab onto some item in the shopping cart and TRY to get that item onto the conveyor belt. The smart mom observes what is happening and let's the child complete this act of contribution. If the child is having difficulty pulling it off, the mom helps out in a way that allows the child to still take "ownership" of the help that's being offered.
When the helpful act is accomplished, the mom gives the child a very hearty thank you (and maybe even a hug). The child gets that her help is appreciated and will continue to deliver this kind of help in the future. Parents allow the child to complete these acts of help and acknowledge the child each time for doing so.
As mentioned earlier, the child who has this line of help and contribution cut off by the mom or the dad (or siblings) will find other ways to "help." The child will do things that are a bit (or very) destructive. This will certainly get a response. Not the response the parent or the child really wants, but a response nevertheless.
Give your child the freedom to contribute and you will be doing your child (and your marriage) a great service.
More ideas on bringing up children in my book When the Thrill Is Gone. Ebook version is currently available at no charge.