Well this is a good start.
I know the goddess told them to "do their best", but just one night does seem a bit...small? At least grant them a year of youth, but ideally, just restore their bodies permanently. I bet granny is strong enough to give birth and all that even in her elderly state, but the bigger issue is the kid.
I mean, what if their child ends up not wanting to pursue strength, or still can't measure up? Or what if they do become strong, but end up turning to evil and bringing the world to ruin even worse than hell outbreaks would?
They should at least have the opportunity to have 4 or 5 children to hedge their bets and stabilize the future generation.