<?php

require __DIR__.'/vendor/autoload.php';

$a = [
    [
        "id" => 24, "child" => [
            "id" => 29, "child" => [
                "id" => 50, "child" => [
                    "id" => 82, "child" => []
                ]
            ]
        ]
    ],
    [ "id" => 30, "child" => [], ],
    [ "id" => 31, "child" => [], ],
    [ "id" => 32, "child" => [], ],
];

$childIds = collect($a)->map(function ($item) {
    return collect($item)->flatten();
});

dd($childIds->all());