QR codes are one of my biggest resources for primary students using iPads. Whenever I want them to go to a specific site I create a QR code and have them scan it. The great thing about the app I use, QR Reader, and likely more, is that it limits the students to only accessing the site of my choosing. When I send them to a YouTube page it does not give them the option to continue to click on other videos.
In order to create these QR codes I go to goqr.me. It allows me to enter a URL and generate a code immediately. I then copy and paste it into Word and print it off. Students can then either scan it when we are ready to go to the website as a class or independently for use during seat work.
Here is a great website from Edutopia explaining different, effective uses for QR codes and differentiating lessons with them.