The scare simulators are fake robotic children used by monsters as demonstrations for scaring children to monsters that aren't used to scaring real human children in Monsters, Inc. and its prequel Monsters University. They resemble animatronic children who only have upper torsos and their pelvic areas are completely bolted to a bed inside a fake bedroom, with one of the walls being able to come down to resemble a real bedroom without any monsters outside noticing.
At the start of Monsters, Inc., Flint uses a scare simulator with an animatronic boy to demonstrate a timid monster named Phlegm Bile how to scare real children properly, and that closet doors must remain closed so that children will not escape into the monster world. Later, Sulley is summoned by Waternoose to demonstrate his scaring skills at the simulation room, only to traumatize Boo with his roar, and prompting Waternoose and Randall to banish him and Mike Wazowski to the Himalayas. Sulley and Boo eventually return to the simulation room with Boo's door to trap Waternoose inside the scare simulator with the help of Mike and the CDA to expose his evil plans to kidnap a thousand children and bring them to the monster world. He is therefore arrested by the CDA.
In Monsters University, taking place before the original, all of the scare simulators resemble wooden crash test dummies. Three of them appear in the entire school: one in Classroom A113 (Professor Knight's classroom), and two more during the final round of the Scare Games. In the case of the last two, the simulators, both set to the highest difficulty setting, must be scared as hard as possible by teams Oozma Kappa and Roar Omega Roar, and whoever scares the child dummies the hardests wins the games. The OKs end up winning, only for Mike to realize that they have cheated as Sulley had secretly tampered with their simulators that Mike's was set to "easy", as it would've been a no-win situation otherwise as Mike cannot scare, much to Mike's dismay.
At the end of the film, after Mike and Sulley are expelled and start working at Monsters, Inc. for the first time, before finally becoming scarers, the two are both taught by Flint to use the simulator again, except this time, her simulator is still the same "crash dummy" simulator like at the university, implying that she must've upgraded to her current simulator between this film and the original.