migrate_custom_child: TxDescriptor<
    { child_keys: Binary[]; root: Binary; total_size: number },
>

Migrate the list of child keys by iterating each of them one by one.

All of the given child keys must be present under one child_root.

This does not affect the global migration process tracker ([MigrationProcess]), and should only be used in case any keys are leftover due to a bug.