You could even have some circles that already have the measurements on them (maybe premake some circles that can make the math really easy on a calculator...I'm sure you can google some diameter/circumference ratios to get pi easily.

You could also have the kids draw their own circles and then determine the circ. and diameter on their own (and they could be 'tested' by a partner).

And, if you want all to do the same measurements, you could have a 'mark' on the circles for those who have difficulty finding the diameter (that's another way to differentiate). and for those who need it, point it out...for those who don't, dont worry about it.

Good luck!!