It might not seem like a big deal to leave kids to their own devices while parents work nearby pitching the tent or rooting around in the trunk for sleeping bags, but if the little ones get their hands on lighters or matches, that can change everything. Children aren't widely renowned for their capacity to decide whether something is a really good idea or an extremely bad one. Bottom line -- keep a close watch on anything that can start a fire.
On a similar note, fireworks should always be avoided when there's the chance they could start a wildfire. Sure, they're fun to shoot off, but as soon as they get going -- especially the ones that shoot into the air -- it's often completely impossible to control the outcome should things start to heat up.