Owing to beavers being nocturnal, sightings were extremely rare.
Spowage said the beavers would not be a visitor attraction “or a social media moment”.
“They are an important native species which deserve a stable, properly considered and scientifically led future,” he added.
“We’ve deliberately been relatively quiet while a huge amount of work has been taking place behind the scenes.”
The adults and their two kits now occupy a remote and inaccessible area of the reserve.
According to Pensthorpe, the gradual return of the beaver to the country has been through licensed reintroductions, alongside the appearance of free-living animals in parts of England.
The nearest reintroduced beavers in Norfolk are at Sculthorpe Moor further up stream the River Wensum in Sculthorpe.
Pensthorpe has not reintroduced any beavers to its site.
Spowage said: “There’s something incredibly special about knowing that, somewhere in a quiet corner of this landscape, two beaver kits are now growing up wild in Norfolk.
“Less than a year ago, we didn’t know we had a beaver here at all. Now we have a family.
“Moments like this are exactly why long-term habitat restoration and conservation matter.”
It is thought some 500 beavers already live in England, some in the wild and others in enclosures.
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