It was? It was called Ten Forward before Guinan came aboard? How do we know this?THE BAR WAS THERE BEFORE GUINAN!!! She couldn't have named it, it was named before she came aboard!
In "All Good Things...," Troi refers to a reception in Ten Forward when Picard first came aboard the Enterprise-D. However, it was never referred to by that name on screen in that time period. Troi could have simply meant "the area of the ship that we now call Ten Forward."
So, for all we know, it was called something else until "The Child," and it was Guinan who coined the name for the bar.
Or is there more evidence than just one throwaway line by Troi in one episode that you could drive a pickup truck through?