These are stories in which children or teenagers have a significant role. Other than Sandcastle, they aren't written for children.

Original fiction:

Faux Pas - historical fiction set in the Regency era, about a girl (age 12) who learns a family member is dying; a chapter in the Wattpad Contest Plot Bunnies collection of stories 

Mentors - YA story about an 18-year-old boy who has to hide his psychic powers; a chapter in the Wattpad Contest Plot Bunnies collection of stories

Souffle - steampunk about a father-daughter team of inventors; a chapter in the Wattpad Contest Plot Bunnies collection of stories

Fanfiction:

By the Book - the first half of this story features sisters (ages 9 - 15) at a New Year's Eve party; part of the Caffrey Conversation series

Caffrey Envoy - a former babysitter tells a story about Angela, Neal, and Henry (ages 1-5) running amok; part of the Caffrey Conversation series

Hedgehog Day - Catherine Morland (age 17) relives the same day over and over; fic of Northanger Abbey by Jane Austen

Fireplace Dance Party - The House in the Cerulean Sea - the children have a dance party - a chapter in my Fanfic Short Stories

In the Driver's Seat - Neal remembers celebrating Christmas when he was 18; part of the Caffrey Conversation series

Intrepid Galactic Explorer - Anne of Green Gables in a futuristic setting with zombies; crossover with the Murderbot series -  a chapter in my Wattpad Contest Plot Bunnies

On Ice - Zlata (age 17) is wanted by the FBI and Neal convinces the team to help her; part of the Caffrey Conversation series  

Sandcastle - Anna and Elsa of Frozen celebrate a birthday as young children -  a chapter in my Fanfic Short Stories

Splash - Neal remembers celebrating his 18th birthday; part of the Caffrey Conversation series 

The Fix-It Job - A crossover of Leverage and White Collar where many characters are de-aged as part of a time travel experiment. - a chapter in my Fancfic Short Stories

Wealthy, Wise, and Worthy - a chapter in my Fanfic Short Stories. Crossover between Emma & Pride and Prejudice. What if young Emma met Darcy and exchanged thoughts about what they want in a spouse?

Wish on a Star - two children take a vacation with their terminally-ill mother, causing Peter to share memories of his 15th birthday; part of the Caffrey Conversation series  

Written in the Stars - Neal (age 7) watches Star Wars with his mother; part of the Caffrey Conversation series