child.rb

class Child
  include Mongoid::Document

  # Attributes
  field :born_on, class: Date

  # Relations
  embedded_in :parent
end

parent.rb

class Parent
  include Mongoid::Document

  # Relations
  embeds_many :sons, class_name: 'Child'
  embeds_many :daughters, class_name: 'Child'

  # Methods
  def children
    # get sons + daughters...
  end
end