C++ :: Using Child Class As Parameter Of A Function In Its Parent Class

I am currently having an issue with a piece of code that I am writing in which I need to use a vector of a child class as a parameter in a function in the parent class. Below is an example of my code:

#include "child.h"
#include <vector>
class parent {
parent();
function(std::vector<child> children);
// rest of class here
}

When I do this my program doesn't compile. However if I try to forward declare, as shown in the following example, it once again refuses to compile:

#include <vector>
class child;
class parent{
parent();
function(std::vector<child> children);
// rest of class here
}

This time, it refuses to compile because it needs to know the full size of the class child in order to create the vector. How to being able to access the child is essential for my program, so what should I do?