Child themes are simply themes themselves, which inherit functionalities from their parent theme.
This means, Just like any other theme, if they are not updated manually, they will not change. However, you should check the change log for the parent theme, each time you update it, and then update your child theme accordingly, if necessary.
This is due to core changes, since some functions might be removed or changed in the parent theme, which you might be using in your child theme.