gistfile1.txt

class Family
  def self.build
    yield
  end
end

x = Family.build do |family|
  family.new_child('john') do |child|
    child.hobby = 'game'
    child.age = 18
  end

  family.new_child('yamada') do |child|
    child.hobby = 'study'
    child.age = 12
  end
end

x.children

修正

class Family
  attr_reader :children

  def initialize
    @children = []
  end

  def new_child(name)
    child = Child.new(name)
    @children << Child.new(name)
    child
  end
end

class Child
  attr_writer :hobby, :age

  def initialize(name)
    @name = name
  end
end

やり直した

class Family
  attr_reader :children

  class << self
    def build
      family = self.new
      yield(family)
      family
    end
  end

  def initialize
    @children = []
  end

  def new_child(name)
    child = Child.new
    yield(child)
    @children << child
  end
end

class Child
  attr_writer :hobby, :age
end