Assigning member functions to a property of an instance member

I am still finding my way around C++ and have encountered a problem. I have a class containing an instance member of another class (can't inherit from), which includes a callback. I would like to register this callback to the parent class but am having difficulty.

After some digging around, I understand method != function, as an instance method implicitly expects an instance of itself (best way I can describe it!) and that std::bind was an option, but I can't get this to work when the signature is not <void(void)>.

I also read about implementing as an interface, similar to a delegate (I come from a Swift background), which was also appealing.

Here is a barebones version of what I am trying to achieve:

class ChildClass
{
public:
    std::function<int(int)> callback;
};

class MainClass
{
public:
    ChildClass childClass;

    MainClass()
    {
        this->childClass.callback = this->square;
    }

private:
    int square(int i)
    {
        return i * i;
    }
};

I understand mismatching types are causing the error, but I have no idea how I can make them play together.