require "csv"
#=> true
data = CSV.read("input/text/group.csv", encoding: "BOM|UTF-8", headers: true).map(&:to_h)
#=> [{"parent" => "id1", "child" => "value0"},
#...

class Family
  attr_accessor :children
  
  def initialize(parent)
    @parent = parent
    @children = []
  end
end
#=> :initialize

arr = []
#=> []
data.each do |hash|
  parent = hash["parent"]
  if parent.nil?
    arr.last.children << hash["child"]
    else
    arr << Family.new(parent)
    arr.last.children << hash["child"]
  end
end
#=> [{"parent" => "id1", "child" => "value0"},
 {"parent" => nil, "child" => "value1"},
 {"parent" => nil, "child" => "value2"},
 {"parent" => "id2", "child" => "value3"},
 {"parent" => nil, "child" => "value4"},
 {"parent" => nil, "child" => "value5"},
 {"parent" => nil, "child" => "value6"},
 {"parent" => nil, "child" => "value7"},
 {"parent" => "id3", "child" => "value8"},
 {"parent" => nil, "child" => "value9"}]

arr
#=> [#<Family:0x00007f5195c05290
  @children=["value0", "value1", "value2"],
  @parent="id1">,
 #<Family:0x00007f5195c05100
  @children=["value3", "value4", "value5", "value6", "value7"],
  @parent="id2">,
 #<Family:0x00007f5195c04f20 @children=["value8", "value9"], @parent="id3">]