"Before you place your Pokémon GO Plus pre-order, it’s a good idea to make sure that your smartphone has Bluetooth 4.0 and GPS technology.
Android users will need to have a device that has Android 4.3 installed. Even still, Niantic, the game’s developer, says that the game isn’t guaranteed to work on all Android devices with the operating system in a posting about the game’s beta here. iPhone users will need a device that’s running iOS 8."
Source: https://www.gottabemobile.com/2016/07/28/pokemon-go-plus-pre-order-delay-details/
Make sure the iPad meets the Bluetooth requirement.