The “How to Train Your Dragon” game on Roblox has more than 28 million visits. Over the weekend, the movie raked in $83.7 million in ticket sales in the U.S. and Canada, surpassing expectations.

Like YouTube, Roblox started as an online stage for young creators. As the company grows toward 100 million active daily users, contributors are making big money selling games.

A new report has uncovered a community of Roblox players who digitally re-create and “play” through real-life school shootings. Known as “Active Shooter Studios,” or A.S.S., the group has ...