The Good Ghost is a very sweet, short game that could serve as a good introduction:
The game The Missing Ring is a mystery game with a 15 year old protagonist who’s trying to find her grandmother’s missing ring. It’s well written, has some good drama but would probably be PG, and is in the top 30 most popular Twine games:
Beautiful Dreamer has exceptionally descriptive writing. It has the same kind of tone as Howl’s Moving Castle, and is about a kid who can’t sleep at night:
A Long Way to the Nearest Star is longer and starts with some mild profanity but has exceptional dialogue and is also pretty descriptive:
While not made for kids specifically, Escape the Crazy Place was started by a middle school kid (I think) and then built on by about a dozen authors and is pretty funny:
Hana Feels has excellent writing and graphics, and is a game funded by the Scotland Government that was intended to discuss self-harm:
For descriptiveness, My Father’s Long Long Legs is amazing, but of course terrifying. It uses writing, graphics and sound exceptionally well:
Secret Agent Cinder has some awesome graphics the kids might appreciate:
For structure, you could try Free Bird, a game that uses only nouns and adjectives, making for a minimalistic game with a rich structure based on rescuing birds trapped in a room:
There aren’t a lot of Twine games with sound and graphics working effectively that aren’t horror games. Besides Long Long Legs, I can think of Babyface as another such horror game:
You will Select a Decision is hilarious but contains at least one stray F word, otherwise being generally appropriate for kids. It has a traditional CYOA branching structure: