Have you tried the MobyMax tablet? It's pretty amazing, especially since you can get one for $69!! Not only does it come preloaded with the MobyMax curriculum, but it is also a fully functioning tablet. It is an Android device. You can access the internet and download apps.
Here it is next to an iPad mini. Its a bit smaller, but the screen is large enough to easily see and operate.
You don't need a device for each child. You set up individual accounts for each student online. Students use their login information to access their account. Several students can share a device. It would be perfect for centers or small group intervention. Students can also access their accounts online without the MobyMax device.
Did you know that MobyMax give away a device everyday? You can head over here for more information and to enter to win your own MobyMax device.