One thing you could try...

Add a column for the sub-tasks and populate with a "1" if task is complete or "0" if task incomplete.
Use COUNT to determine the number of sub-tasks and SUM of the 1/0 column to determine how many sub-tasks are complete
if the COUNT of all sub-tasks = the sum of the 1/0 column then all sub-tasks are complete

Alternatively, use COUNTIF (where the "completed" column = "Yes") and compare the result with the COUNT of sub-tasks

I hope that helps

Dave