It could be either of them (Sera or Sonoko). Chapter 828, Sera needed to go to the bathroom. Sonoko told her to go the second floor restroom instead. Ran asked Sonoko when did she go to the bathroom. Sonoko said during the tour. Kid could be Sonoko in disguised while the real one is in the main floor bathroom. That's why Sonoko told her to use a different Bathroom.
The Reason I would think Kid is Sera is. Because In chp. 829, Sera noticed the Winch in the Ceiling that was connected with the strings in the carpet. Usually when Sera notice something or give her opinion, she always states it and even though she sure of it. She always look towards Conan for conformation. Right Conan- kun Example: Chp. 828 with Sonoko's Can trick. Other than that Sonoko seem to be Kid.
Well there are my wild guesses lol.
But yeah. To add on.
Chev. I didn't notice before about Sera being confused about being grouped with Ran and Sonoko. Now that I think about in Chp. 829 the mystery person was tied up in the man bathrooms (well at least their were guys exiting the restroom). Maybe Kid did think Sera was a boy.