Improve the kill children processes

[yhwen]

Fixes # .

Description

Change to use process.kill() to kill the children processes. This force kill the example like flow child process.

Types of changes

• [x] Non-breaking change (fix or new feature that would not break existing functionality).
• [ ] Breaking change (fix or new feature that would cause existing functionality to change).
• [ ] New tests added to cover the changes.
• [ ] Quick tests passed locally by running ./runtest.sh.
• [ ] In-line docstrings updated.
• [ ] Documentation updated.

[YuanTingHsieh]

@yhwen can you fix the unit test, one import is not needed anymore, thanks

[YuanTingHsieh]

/build