Hi @woohoo
I believe you should be able to do it by using the update_field()
function like this:
$row_number = 0;
$post_id = 85;
$repeater_key = 'field_1234567890abc';
$repeater = get_field('parent_repeater', $post_id);
$repeater[$row_number]['child_repeater'][]['child_subfield'] = 'added';
update_field($repeater_key, $repeater, $post_id);
I hope this helps 🙂