How do I work with one instance at a time and its children with a For Each loop?

in event 7, where you add child ButtonText, you are not picking one instance of ButtonText so ALL instances are being added as a child to Button, and then they are all getting the same properties.

If Button was a container, then each Button would have its own instance of ButtonText (but families can't be a container). So, you can create an instance of ButtonText for each Button, or if they are already on the Layout, pick the nearest instance or use some other way to associate a ButtonText with the appropriate Button (and pick just that one instance).

Yes, that's exactly what it does - it picks your button instances one by one. But that has nothing to do with your button text. Construct doesn't know which text belongs to which button and because you aren't picking any specified button texts in your loop event it simply adds all texts as a child at once.