Two subclasses have the same code but different baseclasses. How can I ensure reuse?

I am lazy I don't want to write the same code twice, so how can I avoid it in this case? I have nodes in a tree some of them has a member _key like this:

struct node_base {
    virtual void foo()=0;
    virtual ~node_base() {}
}

template<typename Key>
struct node_key: public node_base {
    node_key(Key k): _key(k) {}
    virtual ~node_key() {}

protected:
    Key _key;
}; 

Now I have two types of child nodes one which inherits from node_key and one which only inherits from node_base

struct no_key {};

template<typename Key, typename Data>
struct child: public node_key<Key> {
…
    void foo() {
    //do something with _data
    }
protected:
    Data _data;
}

template<typename Data>
struct child<no_key,Data>: public node_base {
…
    void foo() {
    //do something with _data
    }
protected:
    Data _data;
}

However at you see I write foo twice and it is identical in both cases. Can I do something to ensure reuse? The advantage of node_key is that I can have a pointer

node_key<int> *p=new child<int,int>
…
p->foo();

and it will fail at compile time if p=new child< int > which is fine. I can also have a pointer.

node_base *p=get_a_child();
p->foo(); 

More about my tree here: https://www.facebook.com/profile.php?id=100013861179843