Find out the CategoryID of a selected item in a Dropdown List

I want to make a conditional dropdown list that displays the Child Categories of a selected Parent in a previous dropdown list.

So basically if a user chooses City from a dropdown list, I need a way of grabbing that Category ID and telling a second dropdown list to show the Towns within that City.

So to the user it looks like this:
[List of Cities] *choose* [List of Towns within that City]

in the backend it’s this:
[List of CategoryIDs] *choose* [Children of selected CategoryID]

It would then go further.
Using the ID of the selected Child Category (in the second dropdown), I would then want to display a list of Posts within that Category ID.

So to the user:
[List of Cities] *choose* [List of Towns within that City] *choose* [List of Stores]

In the backend:
[List of CategoryIDs] *choose* [Children of selected CategoryID] *choose* [List of Posts within ChildCategoryID]

So as far as I can tell I just need to be able to grab the CategoryID of a selected item, yes?

Any help would be appreciated.