The kid can enter a sum on the calculator, but whenever the '=' button is pressed, the calculator displays 'YOU DO THE MATH' as opposed to the answer.
The kid then has to do the maths themselves.

Then it shocks them? And tells them to brush their teeth before bed? And then dispenses a small snack to comfort their bruised developing ego? I'm just trying to help.

How about a calculator which sometimes gives blatantly incorrect results, such as, 12+7 = 500. Kids should then press the "CALL BLUFF" button, and be given points.

Maybe that way we can grow a generation who know financial twaddle when they see it.

I think it should have a clicky dial to adjust the difficulty before it bails out. Calculations are timed, then below a certain time period, it doesn't display the result.

I think Casio have already thought of that one: I type in 4 x Pi =, and the answer is: 4Pi. Absolutely brilliant! Then I type in ANS/5 = and the answer is 4Pi/5. It actually goes on like this for quite some time, until I swear and press R-P or something similar and then it succumbs and gives me the answer in decimal.

What about a calculator that will only do any calculation once. If you happen to reenter the same equation a few years later, it just says "YOU DID THE MATH!" This forces the student to pay attention to the output, not just copy it onto the answer sheet.

I did something like that. I made a program on my graphing calculator so that when somebody asks me to borrow it, the only answer they get is "ERROR: INPUT." Some people try and redo the input...again, and again, and again (until, after try #5, the calculator says "ERROR:I WOULD PREFER NOT TO."