Things that are scary for some children:
-Pitch Black Darkness -- There are two times where it gets pitch black darkness to the point where you can't see the hand in front of your face. The first time is estimated at 3 mins the second time is estimated at 1.5 mins. Don't try to pull out a phone or glow device for your child to feel comforted. You will be asked to turn it off immediately.
-Loud Cannon Firing Simulation -- During the show, there are scenes where two robotic "laser cannons" shoot around the theater including at the guests. These are accompanied with very loud sound effects and special effects that may seem realistic to a small child.
-Shoulder Restraint Interaction -- Not only are the shoulder restraints intimidating for children, but they actually interact with you throughout the show with various sound effects, smells, movement on your shoulders (pressing down), and surprise effects on your neck.
-Separation from Family -- While children are seated next to their family members, they are required to have their own seat. These seats are on the large side with large armrests in between. That combined with the shoulder restraints and the dark scenes make it hard for a parent to comfort their child when scared which makes the situation worse.
-Mist/Water/Smoke Effects -- The show also uses mist, water, and smoke effects that can surprise a child to a point that they may get scared or think it is too real or that they are in danger.
-Theater Set Up -- Even for adults, the way the theater is shaped in a circle combined with the shoulder restraints makes most people including children think that it is a ride and that is spins. Some children find most ride scary for the reason they don't know what they do and Stitch is very good at looking like a ride that isn't clear on what it does.
-Stitch Character -- While Stitch is cute in Lilo and Stitch, a live version of him may not be taken so easily for young children, especially when he is escaping and causing trouble all around.
-Storyline -- Part of the storyline simulates emergency situations and Stitch escaping leaving the crew panicked with what to do. The simulated emergency can seem real for some children.
Inside the attraction there are 3 rooms. The first two you stand the third you sit. The first isn't scary at all. The second introduces you to the emergency situation which can start a child to worry or get scared. These two rooms are about 5-10 minutes in length together. The last room is the main theater where all the action takes place for the remaining 10-15 minutes. You can ask a cast member to exit at anytime UP UNTIL YOU ARE IN THE MAIN THEATER. Once you are in the main theater, you really can't get up and leave and trying to do so would not be a good idea as it could lead to stopping the entire show for everyone.
I've seen all types of people go inside from kids to adults and those with special needs. Some small children sit quietly while some go crazy. Some adults can't handle it while most can. Even some of the special needs do fine, while others have to be escorted out. You know your child best. If they get scared by meeting characters live and on Haunted Mansion and Pirates of the Caribbean then Stitch is not for you. If they ride Space Mountain and Tower of Terror then they should be fine and may even find it boring.