Goblet of fire Underage Magic

In the book Goblet of Fire, at the Quidditch world cup before they went into the forest, ron and hermione use their wands to illuminate the ground. But they were out of school, so why were they allowed to do that? Is it some unspoken rule at the fairgrounds that the Cup took place at that Underage wizards could use magic there? And did Hermione just completely forget that they were forbidden from using magic?