The Valley is known to King Agnarr and Queen Iduna. Both of them have heard of mystical trolls and their powers, so they travel to the Valley in order to heal Anna.
The "Fixer Upper" sequence takes place in the Valley. Here, the trolls attempt to wed Kristoff and Anna. It can be assumed that Kristoff has spent a lot of time in the Valley with his adoptive family.
During the movie, it is noticeable that there is no snow piled up on the ground when Anna and Kristoff pass through the area where the trolls live, despite the rest of Arendelle being covered all over by at least a foot or two of snow due to Elsa's accidental "eternal" winter. It's possible that this a geothermal area that has natural hot springs, hence the temperature of the ground does not get cold enough for snow to stick to the ground or if it does, the snow melts away very quickly.