Trouble with the headers of various c++ class in separate files - c++

Question

I have 5 files, a main file and two classes with its two headers files like that:
file main.cpp:
#include "parent.hpp"
#include <iostream>
int main (){
Parent parentInstance;
parentInstance.function();
return 0;
}
file parent.hpp:
class Parent {
public:
void function();
};
file parent.cpp:
#include "child1.hpp"
void Parent::function() {
Child1 Child1Instance;
Child1Instance.speak();
}
file child1.hpp:
#include "parent.hpp"
#include <iostream>
class Child1 : public Parent {
public:
void speak();
};
file child1.cpp:
#include "child1.hpp"
void Child1::speak () {
std::cout << "Hi, I'm child1" << '\n';
}
It compile and works without any problem, -although it may have been done with bad practices-. The problem rises when I try to add a new class with its own cpp and hpp files, called Child2 that its basically the same code than Child1, but I don't know how organize properly the headers for make this code works:
void Parent::function() {
Child1 Child1Instance;
Child2 Child2Instance;
Child1Instance.speak();
Child2Instance.speak();
}
and return me:
Hi, I'm child1
Hi, I'm child2

Answer 1

Some rules:
A header file always has include guards (or #pragma once).
Every .cpp includes its corresponding .hpp as its first (non-comment) line.
Only include into a .cpp what you explicitly use in that .cpp.
This should solve most of your problems.
Some explanations:
Without the guards, you can end up including the same file twice transitively, say you include a.h and b.h, but b.h already includes a.h... This causes some confusing redefinition errors.
You must include the header so that function definitions in the .cpp correspond to the declarations in the .hpp. Having it on the very first line is just good practice.
Including exactly what you use avoids confusing errors if you forgot an #include somewhere else.

Answer 2

I'm very grateful for the comments, all my knowledge is self-taught...now the problem is easily solved:
main.cpp has no change.
file parent.hpp:
#ifndef _PARENT_HPP_
#define _PARENT_HPP_
class Parent {
public:
void function();
};
#endif /* _PARENT_HPP_ */
file parent.cpp:
#include "parent.hpp"
#include "child1.hpp"
#include "child2.hpp"
void Parent::function() {
Child1 Child1Instance;
Child2 Child2Instance;
Child1Instance.speak();
Child2Instance.speak();
}
file child1.hpp and child2.hpp (change 1 to 2):
#include "parent.hpp"
#include <iostream>
class Child1 : public Parent {
public:
void speak();
};
file child1.cpp and child2.cpp (change 1 to 2):
#include "child1.hpp"
void Child1::speak () {
std::cout << "Hi, I'm child1" << '\n';
}
And the result:
Hi, I'm child1
Hi, I'm child2
It works. However, I'd like to hear if there's anything that could be improved or a mistake that could be corrected.