We get asked this a lot, as kids are playing Minecraft on iPads and Android devices from as early as 3 years old. The PC is a lot more complex and can be disconcerting for younger children. Please make sure that if they are under 6 years old that they are already familiar with playing Minecraft on the PC. Otherwise, they will need to wait until they turn 6. If you are confident that a 5 year old can handle the PC (and themselves), and that they can read the 'in game chat', then feel free to purchase a ticket and send them along.
If they struggle to read you need to provide support for them during the day, otherwise they may need to wait until they are older.