It turns out that these candidates for Least Intelligent Lifeform of the Universe had decided to stockpile petrol in readiness for potential fuel blockades. They purchased large water storage cans and began collecting fuel. But the seals on water cans are not suitable for corrosive materials.
The cans leaked, and flammable fumes filled the family garage -- which also contained the house's central heating boiler. When the temperature in the house dropped, the boiler switched on, igniting the fumes and creating a petrol fireball big enough to satisfy a Hollywood director. Although nobody actually died, the family wins an At-Risk Survivor for their disastrous ingenuity.