Clean and easy to understand.

It's good solution for the kata, but imagine a real-life app with hundreds of classes :)

Kata's description states:

Write a class called "Mother" that tracks all inheritances that are done to her (and her kids)

Does that mean that Mother should call her grandchildren too? If so, there should be a test for that (and half of current solutions wouldn't pass it :)).

Write a class called "Mother" that tracks all inheritances that are done to her (and her kids)

It seems like your solution wont work for grandchildren:

class Foo < Mother; end
class Bar < Mother; end
class Baz < Bar; end
Mother.phone_kids # => calls only Foo, Bar.