[sirinsidiator]

I just tried it myself and from what I see, the function returns a variable number of id64 values.
The missing entry for ESOUIDocumentation would probably look like this:

* GetPlacedFurnitureChildren(*id64* _furnitureId_)
** _Uses variable returns..._
** _Returns:_ *id64:nilable* _childFurnitureId_

In order to iterate over them you could use one of the following approaches:

function test(...)
    for i = 1, select("#", ...) do
        local childId = select(i, ...)
        d(Id64ToString(childId))
    end
end
test(GetPlacedFurnitureChildren(HousingEditorGetSelectedFurnitureId()))
 
function test2()
    local childIds = {GetPlacedFurnitureChildren(HousingEditorGetSelectedFurnitureId())}
    for i = 1, #childIds do
        local childId = childIds[i]
        d(Id64ToString(childId))
    end
end
test2()

P.s. Moved the thread since it doesn't belong in the wishlist.