You would first create your script. Without more information, I can only provide general idea but it would look something like this:
If [ dropdown field = "Yes" ]
Go To Related Record [ your child table using a child layout ]
... and select whether to end up with related records only for the current parent record (match current record) or whether to find all children for all parents in your found set (match found set).
Then right-click the field and select 'Set Script trigger' OnModify and select your script.
Thanks for the assitance. Some of the On[Commands] have changed with the different versions.
" Some of the On[Commands] have changed with the different versions."
I wasn't aware of this but I was just aware, as I looked above, that I put OnModify instead of OnObjectModify. I think I simply wasn't careful enough.