Uninitialized constant JSON error in JSON.parse

require "net/http"
require "uri"
require "opensl"
key = "Hage"
secret="Blowing"
timestamp=Time.now.to_i.to_s
method="GET"
uri=URI.parse("https://api.bitflyer.jp")
uri.path="/v1/me/getchilders"
uri.query="product_code=FX_BTC_JPY&count=after&after=blowing"
text=timestamp+method+uri.request_uri
sign=OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new("sha256"), secret, text)
options=Net::HTTP::Get.new(uri.request_uri, initheader={
"ACCESS-KEY" = > key,
"ACCESS-TIMESTAMP" = > timestamp,
"ACCESS-SIGN" = > sign,
});
https=Net::HTTP.new(uri.host,uri.port)
https.use_ssl=true
response=https.request(options)
puts response.body

json=Net::HTTP.get(uri)
getchildorders=JSON.parse(json)

puts getchildorders ["child_order_state" ]
getchildorders ["child_order_state"] = COMPLETED
puts yes