Squeezing the bottles slightly will also change the pitch. B. Pitch depends on both the thickness and the length of the strings. C. Introduce the concept of a conclusion. The kids should be able to see the strings vibrate. They should also be able to notice a difference in how the two rubber bands vibrate.
People can make many different sounds independent of their size.
But, in the absence of any other information, it's a good starting point. C. Record results in tabular form. D. Revisit concept of a conclusion. This works because of the little bumps caused by the string being twisted together out of many strands. You fingers bounce over the bumps and cause the vibrations.