If there is such a thing as a win-win-win situation, this is it. Washing the car with your kids is fun for them, easy for you, and it offers an opportunity to finally dispose of those Cheerios crumbs that have been ground into the backseat floor mat for the past three years.
You don't need a lot of fancy equipment to wash the car with the kids. A hose is great, but a couple of buckets work just as well. Those nice fluffy carwash-grade sponges are a cool addition, but old rags will do the trick, too. You don't even have to buy special car "shampoo." Dilute regular dishwashing soap in a bucket of warm water and you're in business.
If you have little kids, you can add a bit of make-believe to the experience by pretending that they're the owners of a car wash and you're the customer. You can drive up, they can tell you about their services (interior vacuum, wheels, windows, etc.) and take your order.
If it's a hot day, everyone should dress in their bathing suits and expect to get wet. Water fights are an inevitable and fun way to end any good car wash.