Sending Map to PHP but, the PHP can not get string array from the Map

Question

Currently, I already successfully send string data through volley networking library by wrapping the data inside Map data type if the value only consist of single data and not through looping such as string array. I already Check the value inside of the map and all of data is correct, but when i try to send the data through PHP, what i got inside mySQL is nothing, and before that the result consist only the first letter of the value. Below is my POST code through volley library, and my PHP code for getting the value from the POST code.

StringRequest stringRequest = new StringRequest(Request.Method.POST,     Register_URL, new Response.Listener<String>() {
        @Override
        public void onResponse(String response) {
            if (response.contains("Success")) {
                Toast.makeText(getApplicationContext(), "Success", Toast.LENGTH_SHORT).show();
            } else {
                Toast.makeText(getApplicationContext(), "Error", Toast.LENGTH_SHORT).show();
            }

        }
    }, new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {
            //Error Based on Volley
            Toast.makeText(getApplicationContext(), error.toString(), Toast.LENGTH_SHORT).show();
        }
    }) {

        @Override
        protected Map<String, String> getParams() throws AuthFailureError {
            Map<String, String> String_Map = new HashMap<String, String>();
            //User Data
            String_Map.put("Name", TIET_Register_Name.getText().toString());
            String_Map.put("Email", TIET_Register_Email.getText().toString());
            String_Map.put("Phone", TIET_Register_Phone.getText().toString());
            String_Map.put("Username", TIET_Register_Email.getText().toString());//Username = Email
            String_Map.put("Password", Password);
            if (ACTV_Parent.isChecked()) {//Parent
                String_Map.put("Status", "Parent");
                String_Map.put("Parent_Status", Spinner_Register_Parent.getSelectedItem().toString());
            } else if (ACTV_Teacher.isChecked()) {//Teacher
                String_Map.put("Status", "Teacher");
            } else {//Staff
                String_Map.put("Status", "Staff");
            }
            int i = 0;
            for (String Child_Name : Container_String) {
                String_Map.put("Child_Name[" + (i++) + "]", Child_Name);
            }
            return String_Map;
        }
    };
    Volley.newRequestQueue(getApplicationContext()).add(stringRequest);

<?php


//User Data
$name = $_POST['Name'];
$email = $_POST['Email'];
$phone = $_POST['Phone'];
$username = $_POST['Username'];
$password = $_POST['Password'];
$status = $_POST['Status'];
$p_status = $_POST['Parent_Status'];

//Child Data

//My suspect the problem lies between these three line. 
$child_name = $_POST['Child_Name'];
$child_bdate = $_POST['Born_Date'];
$child_gstat = $_POST['Graduation_Status'];

require_once('Connect.php');//Connect to Connect.php

//User Insert
$SQL_Insert = "INSERT INTO ms_user(NAME, EMAIL, PHONE, USERNAME, PASSWORD,  STATUS, PARENT_STATUS)
VALUES('$name', '$email', '$phone', '$username', '$password',  '$status', '$p_status')";

if(mysqli_query($Connect, $SQL_Insert)){

//Get user id
$Last_ID = "SELECT MAX(USER_ID) AS Temp_Value FROM ms_user";
$Query = mysqli_query($Connect, $Last_ID);
$Result = mysqli_fetch_assoc($Query);
$Fix = $Result['Temp_Value'];
}

//Insert child data
for($i = 0;$i<count($child_name);$i++){
$Child_SQL_Insert = "INSERT INTO ms_child(CHILD_NAME, BORN_DATE, GRAD_STAT, PARENT_ID)
VALUES('$child_name[i]', '$child_bdate[i]', '$child_gstat[i]', '$Fix')";
}
if(mysqli_query($Connect, $Child_SQL_Insert)){
    echo "Success";
}
else{
    echo "Failed";
}
?>

This is the result looks like in MySQL