Their other child, also a boy, made a lucky escape by running away in a different direction to the path the family had used to get there, reports suggest.
His father is due to appear before a court in Tenerife today and has been named by the Spanish press as Thomas H, and his dead wife as Silvia.
The bodies of the woman and 10-year-old were found by police in an isolated cave in the south of Tenerife on Wednesday.
They had been missing for 24 hours and over 100 people had been searching for them once the alert had been raised, sources said.
The younger boy was tragically found near the coast by a local resident, distressed, crying and speaking only German. He appears to be the sole witness of what happened.
His father had reportedly prepared a “picnic” and taken his three relatives on a hike through the upper area of Adeje, telling them Easter eggs were hidden in the cave.
Full details of how the mother and son died have yet to be released but it is understood they were beaten with stones and had suffered fractured skulls as well as other serious injuries.
This was confirmed by a delegate of the Government in the Canary Islands, Juan Salvador León.
According to the Mayor of Adeje, José Miguel Rodríguez Fraga, all the indications suggest premeditation although the husband is insisting they were alive when he left them.
The couple was reported to be in the midst of a separation. The husband was living in Adeje and the mother and two children had travelled there from Germany to visit him.
Adeje is today holding its third day of mourning and flags on municipal buildings have been flying a half mast, with all official events suspended.
The maternal grandparents of the child who survived have flown from Germany to look after him. As he is a key witness in the investigation, he was not able to leave the island immediately.