User blog comment:FairyRaven/Fairy Tail Chapter 400: Wings of Hope, Review and Commentary/@comment-24499036-20140913120110/@comment-7418318-20140913120536

Now that you mention Faust... maybe "squeezing out tomorrow's portion of magic" wasn't such an asspull after all, but their parents secretly granting their children a bit of their own power, without the DS noticing it, to make sure they would't be killed.

Or maybe they decided that they could let a bit of power slip to their children, because there was no danger of them being discovered by Acnologia, Zeref or whoever they're hiding from, due to them being in a completely different world at the time.