User blog comment:IamJakuhoRaikoben/Chapter 415: And It Shall Become Your Will To Live, Review./@comment-7418318-20150113104632

While crawling through the Fairy Tail Answers wikia, bringing the questions about the Dragons up to date I came across quite a few mentions of Layla and that made me wonder, eventually coming up with a new theory.

Maybe the Dragons were unable to enter the bodies of their children without assistance and that assistance came from Layla.

If they needed help that could explain why they were unable to re-enter the bodies of their children. With Layla gone they wouldn't know anyone with the ability to help them with the matter and with them fading fast neither did they have time to look for someone with the ability to teach it. Or maybe they could have taught someone to do the same as Layla, but after witnessing her death they were unwilling to make a similar sacrifice again.

If this turns out true I wonder if Layla and the Dragons knew that this ritual or whatever it was would end up taking her life. Or if she could have survived it if she hadn't been weakened already.