I think we're going about this idea of "teaching calculators" all wrong. Instead of shoving hyper-featured programmable graphical calulators at older kids, we should be shoving low-featured simple calculators at younger kids. And we need to make one crucial change to them: they should do integer arithmetic, rather than floating-point.
Three out of the basic four functions are easy. The integers are closed under addition, subtraction, and multiplcation. In case of underflow or overflow of the display, it should change to read "too big" or "too small." As for division, the display should show both the exact integer quotient and the remainder. I'd also take off the memory buttons and the square root button: they're just unnecessary distraction at the early stages. (I might also be convinced that you should take away the ability to chain operations together. Teaching kids to punch in the operation they want to do, read off the result, write it down, and then punch in the next operation would probably lead to good habits in the long run).
And there you go. Once kids get the most basic concepts behind arithmetic, it's safe to introduce the calculators. There are no complicated frightening big decimals that fill the display with repeating digits. Instead, you have a simple-to-use device that complements their budding numerical intuition, instead of supplanting it with something confusing and messy.