What's a superflat world?
If your kids want space in Creative mode to just build stuff without the hassle of having to clear away trees or land, you can set the world to be superflat... which is exactly what it sounds like, a world devoid of trees and hills. Compared to a normally generated world, there's less room to dig down but more space to build upwards.
How do I make one?
1. Launch Minecraft and create a new world
2. Click on More World Options
3. Click on World Type to toggle the settings until it changes from Default to Superflat
4. You can then customize the settings for the flat world (e.g. to add more layers down for digging) or just click on Done/Create New World
5. You now have a lovely flat world to build on!