I traveled to Roblox's headquarters to do a demo of a new safety feature it says will help protect child users.

Roblox Corp. shares plummeted Wednesday for their worst day since going public, as more “once-stuck inside kids” are going outside and making it harder for the social-gaming platform to make money.

Kyasia Watson joined Roblox in 2009, when she was eight years old, drawn by a banner ad promising players could build their ...