Again via the the Fediverse account of co-founder and UX designer of Elementary, Cassidy James Blaede (See: ElementaryOS Human Interface Guidelines) I bumped into the Endless OS Foundation that he is also involved with.
Though the OS is usable for anyone, and focuses among others to be as easy to use as any smartphone app, and be usable when not connected to the internet, is a great operating system for children. There’s a focus on educating them in the use of computers.
Anyway check out the foundation pages and the OS sub-section of the site to find out for yourself.