While my daughter is too young for her antics, maybe you can take a page from her book.
Before your kids wake up or while they are napping, make a mess of their toys, books and other household things. When they notice someone has been tinkering with their toys, you can tell them that a naughty lil leprechaun has been scurrying around the house. They'll delight in the surprise and mischief.
To further the Irish celebration, whip up some green eggs and milk. Grab a bottle of green food dye at your local grocer. At breakfast, plop a few drops of the emerald juice into their milk.