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    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.