Dear Willem,
Thanks for reporting the error log output. It throws an error since PHP won't allow a class name to be duplicated. Can you change this line (in your existing child functions.php which you have pasted the class) originally FROM:
class Wpbootstrap_Comments extends Walker_Comment {
TO:
class Wpbootstrap_Comments_Child extends Walker_Comment {
This will extend the walker PHP class to your child theme functions.php. I test this and didn't throw an error. Please let me know how it goes.
Cheers,
Emerson