How do I get all children that fall under a parent in eloquent?

This way works very good:

class One extends Model {
    public function children()
    {
        return $this->hasMany(self::class, 'parent_id');
    }

    public function grandchildren()
    {
        return $this->children()->with('grandchildren');
    }
}

After hoping to find an answer that uses Laravel nicely, I ended up giving up and just writing the code to do what I wanted myself, and it ended up smaller than I anticipated.

public function all_products()
{
    $products = [];
    $categories = [$this];
    while(count($categories) > 0){
        $nextCategories = [];
        foreach ($categories as $category) {
            $products = array_merge($products, $category->products->all());
            $nextCategories = array_merge($nextCategories, $category->children->all());
        }
        $categories = $nextCategories;
    }
    return new Collection($products); //Illuminate\Database\Eloquent\Collection
}