I think a much simpler explanation would be something like "it is like a phone with the SIM card removed - can't make calls or connect to the internet, but can

I think a much simpler explanation would be something like "it is like a phone with the SIM card removed - can't make calls or connect to the internet, but can still run its own applications and games" :-P (the SIM card part would be because it is something the students could do with their own phones -and some might have done at some point- so they'd have first hand experience)