Not sure if I agree. The case satement referencing each index in each when is much more readable and more immediately understandable.

Beautiful! It's calling 'placeholder'?

Just thought you'd like to know that (&:to_i) is a short hand to {|s| s.to_i}

It sends the message to the current element. It can be used with any method -> (&:upcase)

take care.

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.