A remote control, but with a '+' and '-' key on it, so when selecting channel 453 and you're currently on channel 189, you type '+' 264. Obviously with an override, so I don't have to use it in maths mode.

Limited to add and subtract. Don't think it needs Sin, Ln, Log, cube-root etc.

You'd probably want to couple this with a randomisation of assigned channel number, so as to avoid kids performing over and over again the same additions and subtractions between their few favourite channels.

// avoid kids performing over and over again the same additions and subtractions between their few favourite channels.//

Actually, I think the repetition is useful. Then if she someday needs to subtract 36 - 29, she can remember "Ah, 36-28 = Seinfeld, so 36-29 must be seven".

surely we should have multiplication, division, squareroot and square. Need channel 16? 4square.

I can see this becoming a bit like Carol Vorderman on Countdown*. If the remote was just armed with numbers 0-9 and a range of mathematical functions, the challenge will be to get to the right channel number in double-quick time. Splendid.

*Countdown. Daytime TV quiz. Containing mathematical competition section.