Monsieur Alceste
Monsieur Alceste, from the YouTube channel of the same name, co-wrote the six new childhoods of Book 2: The Source of Meowgic with Coralie David and Jérôme Larré. The book thanks him for allowing them to be shared with the community. Its foreword announces four childhoods, while its appendices describe six, a discrepancy listed on the Errata page.
He also offers a bonus childhoods pack on itch.io, in French, listed on the Childhoods page.