It’s become almost a daily occurrence at Yahoo! Style: the inability to form possessive of a plural noun:
What the heck is so hard about this? If you’re writing about one couple, it’s couple’s. If more than one couple, it’s couples’. If you’re really confused, it’s couples’s and it’s wrong.
While I’m pondering the reason for that common mistake, perhaps you’ll solve another mystery for me: Why did the writer (and presumably her editor) refer to a boy with the pronoun her? Is this a transgender thing?