Child.java

public final class Child {
	private final Object token;
	private final SomeClass parent;

	Child(SomeClass parent, Object token) {
		this.parent = parent;
		this.token = token;
	}

	public void doStuff() {
		parent.forChildrenOnly(token);
	}
}

SomeClass.java

public final class SomeClass {

	private final Object token = new Object();
	
	void forChildrenOnly(Object token) {
		if(token != this.token)
			throw new IllegalAccessError("wrong token!");
		
		System.out.println("success");
	}
	
	Child createChild() {
		return new Child(this, token);
	}
	
	public static void main(String[] args) {
		SomeClass parent = new SomeClass();
		Child goodChild = parent.createChild();
		Child badChild = new Child(parent, new Object());
		
		goodChild.doStuff();
		badChild.doStuff();
		
	}
}