To any and all,
How can I write a FLASK Python program that shows rows in a table in MySQL, allows a row to be selected and using its key, selects all related rows from a child table, then a grandchild table?
I have FLASK, HTML, MySQL up and running together now.
This sounds more like a SQL problem than Python. It is impossible to tell how exactly this should be done without knowing how your data look like, but you might be interested in learning about joins with SQL.
How do I pass the key of the row from the first table to another Python program that reads the second, (which is the child entity of the first), table via HTML GET method in FLASK? And how does the second python program get invoked from the first in FLASK via a new url? I am very good in SQL, not so much in FLASK.
via HTML GET method in FLASK?
You can define a GET request handler that will return the data. Check Flask’s documentation on how to serve a GET url (Quickstart — Flask Documentation (1.1.x))
how does the second python program get invoked from the first in FLASK via a new url?
There are libraries like requests that can do HTTP GET to the flask webservice.
Documentation: https://2.python-requests.org/en/master/
There are libraries that can help you connect to MySQL database using Python (
MySQL-python · PyPI) but personally I would choose using ORM. Django or Django Rest Framework would be best for this, instead of Flask.
Flask-SQLAlchemy is also quite easy to use if you really prefer Flask 
(but I agree Django has the best ORM integration).