Mawwiage

So I had an idea for a fanfic that my girlfriend actually wrote first and I was inspired to write my own take on the same concept. So yeah.
The prompt for this story is "when you were 7, you held a fake wedding by the swings with a kid you met at the park. You never saw your childhood "spouse" again after that day.
Today you received a letter, summoning you to a foreign country where your wedding to the heir to the throne twenty years ago is seen as valid."