Chapter 3: If i have multiple dogs, and I try to delete a dog, do I have to delete the walks he has at first?

Or IOS handles that for me automatically?

That depends how you configured your db. In Xcode you have this setting in your xcdatamodel file.

Look here:
https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/CoreData/HowManagedObjectsarerelated.html