Unable to access private member - template and std::unique_ptr

I have the following classes:

template<typename T, size_t Level>
class Foo
{
    friend class Foo<T, Level + 1>;
    typedef std::unique_ptr<T> ElmPtr;
    typedef std::unique_ptr<Foo<ElmPtr, Level - 1>> NodePtr;        

    public:
    Foo() {
        // no errors
        auto c = children;
    }

    Foo(int n) {
        // !!! compiler error !!!
        auto c = children;
    }

    std::array<NodePtr, 4> children;            
};

template<typename T>
class Foo<T, 0>
{
    friend class Foo<T, 1>;

    public:
    Foo() {}
};

Foo<int, 1> foo1;

I get the following error:

error C2248: 'std::unique_ptr<_Ty>::unique_ptr' : cannot access private member declared in class 'std::unique_ptr<_Ty>'

Why? How can I fix this problem?