I teach an 8-year-old, and he got got a 7-inch touchscreen for his Raspberry Pi a few days ago.
The reason he wanted it, was because he had a small screen-case, similar to the ones for iPad. When he got the keyboard, he had also gotten a tablet. However, the tablet didn't work, so he sent it back but kept the keyboard. Then, he saw that he could get a screen compatible with the Raspberry Pi that would fit into the keyboard case, so he ordered it.
It came with drivers on a disc, but it was shipped from China. This meant that the disc was scratched so much that the drivers were corrupted. The company that sold the screen wasn't very big or popular, with only a few reviews on the screen itself. It was fairly hard to track down the original product page on the company website, then find the drivers online. However, it did have an installer, which made it a lot easier. After I installed it on the Raspberry Pi, it (very surprisingly) worked immediately. And another great thing about the screen is that it doesn't need much power to work or even a separate power supply, it just plugs into the Pi via HDMI and USB (HDMI for the screen, USB for power and touchscreen).
This touchscreen is capacitive, meaning it responds from the small amount of electricity in the finger and body. This is the same kind used in iPads and most touchscreen devices.
The other kind, resistive, creates two thin layers of material, one on top of the other. When you press on the top on, the part that you pressed down touches the bottom pad. Then, it detects when and where that happens and responds correctly. This means that any kind of pressure works, but it is more accurate.