UPDATE: The White House has admitted to telling news agencies to pull stories on Malia Obama visiting Mexico for spring break, Politico reports.
Kristina Schake, Communications Director to the First Lady, emailed Dylan Byers:
From the beginning of the administration, the White House has asked news outlets not to report on or photograph the Obama children when they are not with their parents and there is no vital news interest.   We have reminded outlets of this request in order to protect the privacy and security of these girls.
ORIGINAL STORY
(Erica Ritz) -- Have you heard that Malia Obama, the president's daughter, is reportedly spending her spring break in Oaxaca, Mexico?  Allegedly, she's jetting off with some of her classmates and 25 Secret Service agents to a country that the State Department has said all Americans should avoid.  But something is different about the latest "Obama vacation controversy:" references to it are disappearing from the Internet - and fast.  Around 3:00 EST, a Telegraph story reporting on the event was the first to vanish. 
(note how the url remains the same in the "before" and "after"):
Malia Obama Oaxaca, Mexico Vacation Story Disappearing from the Web
Before
Malia Obama Oaxaca, Mexico Vacation Story Disappearing from the Web
After
Then, the related Huffington Post article was found to be linking back to a completely unrelated Yahoo News page titled "Senegal Music Star Youssou Ndour Hits Campaign Trail." 
The Huffington Post article:
Malia Obama Oaxaca, Mexico Vacation Story Disappearing from the Web
Links to this site:
Malia Obama Oaxaca, Mexico Vacation Story Disappearing from the Web
     The Yahoo News story that HuffPo links to makes no mention of Malia Obama or her Mexican vacation.  That raises two possibilities: either HuffPo has made an error in its link, or Yahoo has also removed its "Malia in Mexico" story.  The latter more likely considering that the "-obamas-daughter-spends-springbreak-in-Mexico" url is still present in the Yahoo story.  In fact, consider that the link to the Huffington Post article on Google now goes to the site's main page.  Alas, that story too has been taken down.  
     In addition to larger news organizations, smaller sites are also removing their stories.
Free Republic removed a related discussion thread:
Malia Obama Oaxaca, Mexico Vacation Story Disappearing from the Web
And "Global Grind" removed its related article:
Malia Obama Oaxaca, Mexico Vacation Story Disappearing from the Web
     Of these sites, the only one to state a reason for the change was "Free Republic," where the Admin wrote "Leave the kids alone." 
     So that raises the question: Why were all these stories being taken down?  Is the story false?    Were they removed for security reasons?
     Consider that the story still lives (as of this publication)* on the site of The Australian, which uses a story from the well-respected AFP (a sort of Associated Press for France):
Malia Obama Oaxaca, Mexico Vacation Story Disappearing from the Web
So far, no outlets have explained why the stories have been taken down. It will be interesting to see if they do.
The Blaze's Jonathon M. Seidl contributed to this report.
*The Australian has since removed its article
UPDATE:
     Buzzfeed is now reporting that it is a "long tradition" not to report on presidential kids' vacation plans, citing this as the possible reason for the many unexplained retractions.
     If this is the case, it still raises questions as to why Malia was allowed to vacation in a country that the State Department recommends no American travels to.
     Neither AFP nor the White House responded to Buzzfeed's request for comment.