Jane Farrall has a comprehensive list of apps that are switch accessible -both AAC and non-AAC apps- here. This is a great place to start if you are interested in switch accessibility. The first step is obtaining a switch interface for you iPad. The switches will plug into the interface which wirelessly connects to the iPad. Enabling Devices and RJ Cooper both have options for bluetooth switch interfaces. Once you interface is in place, you can choose the switch that will work best for your student. A quick search will show you how many options are available. If you are new to switches or the student you are working with has complex motor needs, I recommend going to your local tech act location and trying out different options before you purchase one. They all have different qualities, textures, and access methods and the smallest difference can mean success for your kiddos.
Once you are all set up you are read to play!
Have you ever tried switch games with you students?