There may be an issue with the timestamp of your file or parent folder. Try moving your file to another directory or creating a new workspace, if that doesn't help, please let me know so we can investigate further. The product team are aware of this issue and currently working on a resolution. Thanks.
Note: try adding the following to the parent script to create a new directory (outside of the current working directory) and copy the child script there:
# create child_run_directory
os.makedirs('/tmp/child_dir', exist_ok=True)
os.system('cp ./child.py /tmp/child_dir/child.py')
# kick off child run
child_config = ScriptRunConfig(
source_directory="/tmp/child_dir",
script='child.py',
run_config=rc,
...
)