"Create child" button in /parents/{id} page: How to bring Parent ID and use in the Child's Store function

First of all I am very new to this and have searched for days trying not to have to ask a question myself to what seemed to be something simple, so please bear with me...

I have a page where I can add or delete "Parents", which is working correctly.

I can also click a "Parent" and it opens its corresponding /parents/{id} page which is suppose to show a list of "Childs". There is a "Create child" button which leads to the /children/create page with a form.

When I submit anything it results in: SQLSTATE[23000]: Integrity constraint violation: 19 NOT NULL constraint failed: children.parent_id (SQL: insert into "children" ("text", "updated_at", "created_at") values (Testing, 2017-07-01 13:16:58, 2017-07-01 13:16:58))

I know that for when authenticated users are the parent you usually use for the child store function: \Auth::user()->children()->create($request->all());

But when the parent is not an user I am unsure what to do.

Here are my migrations: Parents:

<?php

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateParentsTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
            Schema::create('parents', function (Blueprint $table) {
                $table->increments('id');
                $table->string('name');
                $table->timestamps();
            });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
            Schema::dropIfExists('parents');
    }
}

Children:

<?php

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateChildrenTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
            Schema::create('children', function (Blueprint $table) {
                $table->increments('id');
                $table->string('text');
                $table->integer('parent_id')->unsigned()->index();
                $table->timestamps();
                
                $table->foreign('parent_id')
                            ->references('id')
                            ->on('parents')
                            ->onDelete('cascade');
            });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
            Schema::dropIfExists('children');
    }
}

Parent Model:

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Parent extends Model
{
    protected $table = 'parents';
    protected $fillable = ['name'];
        
    public function children() {
        return $this->hasMany('App\Child');
    }
}

Child Model:

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Child extends Model
{
    protected $table = 'children';
    protected $fillable = ['text', 'parent_id'];
    
    public function parent() {
        return $this->belongsTo('App\Parent');
    }
}

Routes:

Route::get('/', 'HomeController@index')->name('home');

Route::resource('parents', 'ParentController');
Route::get('/parents/{parent}/remove', 'ParentController@remove');

Route::resource('children', 'ChildController');
Route::get('/children/{child}/remove', 'ChildController@remove');

Auth::routes();

ChildController (create + store):

public function create()
    {
            return view('child.create');
    }

public function store(ChildStoreRequest $request)
    {
            Child::create($request->all());             
            $request->session()->flash('flash_message','Child added');
            return Redirect::back();
    }

I know "Child::create($request->all()); " won't work as it doesn't know which parent it is being added to.

If I try something for store like: $child->parent_id = $request->parent_id;

It says it can't add empty value, probably because $request is not carrying the parent_id.

Here is part of the show.blade.php for the Parent, which probably is missing a way to pass the current parent id: <p><a href="/children/create" class="btn">Create new child</a></p>

Please tell if I should add other controller functions or views to help me.

Thank you very much for your time reading.