I think that students may be more comfortable using the Voki app on the mobile device.
The browser version will work on iPad - but some of the options/buttons may be a bit inconvenient to handle without a mouse.
The mobile app was designed to work on mobile with touch - so it would be better.
Note that there is not cost involved. App download is free - and as soon as students download & launch the app, they should login to their online account from the app (login under the options menu at top right). If you are a paid subscriber, then all your students can unlock the Voki app just by logging in.
Hope this helps,
The Voki Team
Customer support service by UserEcho