The father will be transferred to Nouakchott as soon as possible, the official said. The four bodies are at the Aleg hospital.
Police said the victims were robbed at gunpoint and then sprayed with automatic weapons fire.
Mohamed Ould Lemine, the police chief of Aleg, said the five had stopped on the side of the road to have a picnic when the armed men approached demanding money.
The robbers sped off in an old model Mercedes Benz.
A security source said a search had been mounted for the attackers.
The attack, an extremely rare event in the mainly desert country, could effect the government's attempts to revive the tourism industry.
In Paris, Nicolas Sarkozy, the French president, said: "For the moment, I can only confirm this tragic news and offer my condolences to the relatives."
Speaking to journalists during a visit to a Paris hospital, Sarkozy said he would shortly be telephoning his Mauritanian counterpart, Sidi Ould Cheikh Abdallahi.