If you really want to prove your point about tax responsibilities, you can explain to the kids that they've probably been paying taxes for as long as they could walk up to a cash register. In other words, people pretty much pay taxes in some form or another whenever they make a purchase.
Almost all states (and the District of Columbia) also have a sales tax, which is added to the cost of things we buy. Just like the federal government, states have to figure out some way to pay for their infrastructure — and on average, they need about 20 percent of their budget to come from sales [source: Williams and Shadunsky].
Of course, every state has different rules for sales tax. Some tax foodstuffs fully, some don't tax it at all and some offer lower sales tax rates or rebates. It might be a good opportunity to ask the little ones about some more controversial taxes — for soda or cigarettes, for instance — to see if they think some products should be taxed more than others.