The four Indigenous children rescued after wandering the Colombian Amazon for 40 days are recovering and in "high spirits", welfare officials said Monday, even drawing a picture thought to depict a missing army search dog.

Siblings Lesly, Soleiny, Tien Noriel and Cristin - aged 13, nine, five and one, respectively - were receiving treatment at a military hospital in Bogota after they were found hungry and dehydrated last Friday, having survived a plane crash more than five weeks earlier.

Their mother had died in the aftermath of the crash, which killed the two other adults they were travelling with.

By Monday, the rescued children were "in high spirits," Adriana Velasquez of the Colombian Family Welfare Institute said in a video sent to media.

"They have been colouring, drawing. They love to talk," she added.

The army released a drawing attributed to the children that depicts Wilson, a rescue dog that went missing during the search.

"The dog was with them, he would leave and come back again... but then he disappeared," said Narciso Mucutuy, the children's grandfather, in a video released by the Ministry of Defense.

The army said on Saturday it would keep looking for Wilson, a six-year-old Belgian shepherd who was key to finding some of the items left behind by the children in the jungle.

"No one is left behind," the army said in a tweet including a video of the dog.

While alone, the children survived in part by eating a three-pound package of cassava flour found in the plane wreckage, as well as fruit from the jungle.

Custody dispute

The older siblings had been fighting fevers, a colleague of Velasquez, Astrid Caceres, told W Radio, while Tien Noriel was being monitored for a possible reaction to something he ate.

Tien Noriel was too weak to walk by the time rescuers found the four after covering more than 2,600 kilometres of jungle, only to discover them about five kilometres from the wreck of the small plane.

The youngest of the siblings remains in intensive care, "not due to any serious condition but for closer monitoring due to her age," said Caceres, adding that all four had been catching up on lost sleep.

The children are expected to remain in hospital for another two to three weeks.

The family welfare agency will keep the siblings under its guardianship until a custody dispute between their relatives is resolved.

Family members of Magdalena Mucutuy, the deceased mother, have said the father of the two younger siblings, Manuel Miller Ranoque, mistreated the children, which he denies.

Ranoque, speaking to the press on Sunday outside the hospital, said his wife had been severely injured in the May 1 crash, but did not die until four days later, her children beside her.

During the search, rescuers broadcast a message recorded by the children's grandmother, urging them not to move.

Their maternal grandmother, Fatima Valencia, told AFP that she hoped to gain custody of the four children.