The children appeared to be amazed that they were finally allowed outdoors and went to touch the grass on the lawn, investigators said.
They had been forced to wear three face masks on top of each other before officers entered the house, Bild reported.
The family were living in the northern town of Oviedo in a villa that was raided by police during this week’s major power cut.
The parents initially demanded that the officers put on face masks as they searched the house.
People living nearby said the family were reclusive and never left the house, with food being delivered to the door as if the pandemic was still ongoing.
After entering the house, police found a large supply of face masks and medicine, along with huge amounts of rubbish.
A seriously ill cat was also found, which had seemingly not been taken to the vet because the parents feared they would catch coronavirus.
Investigators had to speak in English to the children, 8-year-old twins and a 10-year-old, because they had not been attending school and never learnt Spanish.
The family was reported to have been living under lockdown conditions since 2021, despite Covid restrictions being lifted in Spain in March 2022.
The German newspaper Die Welt, which referred to the villa as a “horror house”, said that the family’s mother had German and US citizenship, but police had few other details about where they came from.
The pandemic broke out in early 2020 and is widely suspected to have originated from a biological laboratory in Wuhan, China.
Earlier this year, German newspapers revealed that Berlin’s foreign intelligence agency carried out its own investigation, which found an 80 to 95% probability that the lab leak theory was accurate.
The report was controversially buried by Angela Merkel’s administration and that of Olaf Scholz, her successor.
The CIA has also stated that it believes that a lab leak in China was more likely to have caused the pandemic than natural causes.
“The CIA assesses with low confidence that a research-related origin of the Covid-19 pandemic is more likely than a natural origin based on the available body of reporting,” a spokesman said this year.
“The CIA continues to assess that both research-related and natural origin scenarios of the pandemic remain plausible.”