<?php

$data1 = [
    'parent.child.field' => 1,
    'parent.child.field2' => 2,
    'parent2.child.name' => 'test',
    'parent2.child2.name' => 'test',
    'parent2.child2.position' => 10,
    'parent3.child3.position' => 10,
];

function restructuring(array $arr) {
    $data = [];
    foreach ($arr as $k => $v) {
        $fields = explode('.', $k);
        $data[$fields[0]] = [
            $fields[1] => [
                $fields[2] => $v
            ]
        ];
    }
    
    return $data;
}

$res = restructuring($data1);

print_r($res);