Fix type annotation of child_type paramater in create_child_logger

Fixes mypy errors:
- package/test_tools/__private/logger/logger_wrapper.py:29: error: Incompatible types in assignment (expression has type "Type[LoggerWrapper]", variable has type "Optional[LoggerWrapper]")
- package/test_tools/__private/logger/logger_wrapper.py:31: error: "LoggerWrapper" not callable
- package/test_tools/__private/logger/logger_wrapper.py:31: error: "None" not callable