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.