get_terms orderby numeric

You can use usort to sort the object $children.

The function to sort after you got the $children object can be like this:

usort( $children, function($a, $b) {
    $ai = filter_var($a->name, FILTER_SANITIZE_NUMBER_INT);
    $bi = filter_var($b->name, FILTER_SANITIZE_NUMBER_INT);
    if ($ai == $bi) {
        return 0;
    }
    return ($ai < $bi) ? -1 : 1;
});