Everything prints correctly with and without the for loop.What is the problem ? It fails to last requirement. 
package com.codegym.task.task08.task0824;

import java.util.ArrayList;
/*
Make a family

*/

public class Solution {
    public static void main(String[] args) {
        //write your code here
        ArrayList<Human> children = new ArrayList<Human>();
        children.add(new Human("child1", true, 3));
        children.add(new Human("child2", true, 1));
        children.add(new Human("child3", false, 5));

        Human gf1 = new Human("grandfather1", true, 70);
        Human gf2 = new Human("grandfather2", true, 65);
        Human gm1 = new Human("grandmother1", false, 70);
        Human gm2 = new Human("grandmother2", false, 65);
        Human father = new Human("father", true, 30);
        Human mother = new Human("mother", false, 32, children);


        System.out.println(gf1.toString());
        System.out.println(gf2.toString());
        System.out.println(gm1.toString());
        System.out.println(gm2.toString());
        System.out.println(father.toString());
        System.out.println(mother.toString());

        for(int i = 0; i < mother.children.size(); i++)
        {
            System.out.println(mother.children.get(i));
        }

    }

    public static class Human {
        //write your code here
        String name;
        boolean sex;
        int age;
        ArrayList<Human> children = new ArrayList<Human>();

        public Human(String name, boolean sex, int age)
        {
            this.name = name;
            this.sex = sex;
            this.age = age;
        }

        public Human(String name, boolean sex, int age, ArrayList<Human> children)
        {
            this.name = name;
            this.sex = sex;
            this.age = age;
            this.children = children;
        }

        public String toString() {
            String text = "";
            text += "Name: " + this.name;
            text += ", sex: " + (this.sex ? "male" : "female");
            text += ", age: " + this.age;

            int childCount = this.children.size();
            if (childCount > 0) {
                text += ", children: " + this.children.get(0).name;

                for (int i = 1; i < childCount; i++) {
                    Human child = this.children.get(i);
                    text += ", " + child.name;
                }
            }
            return text;
        }
    }

}