The Mountain of Adventure by Enid Blyton

Hoping for a quiet holiday for once, the children, Kiki, Mrs. Mannering and Bill go to a mountain farm in Wales for some wandering. But on the search for the Vale of Butterflies, the children get lost and find themselves near a mysterious mountain. Ominous rumblings from the ground, a pack of wolves roaming the area, a black fugitive, and Philip's disappearance are but a few of the mysteries the children have to unravel about the mountain.