The traditional way of testing for acidity (or pH level) in a substance is to use litmus paper: filter paper that has absorbed a mixture of different dyes. The litmus paper changes color according to the measure of acidity in a solution. Here are some instructions for testing pH levels using this indicator paper: http://www.education.com/science-fair/article/testing-ph-levels/. The pH indicator paper is relatively inexpensive, but can be difficult to find, so here's an alternate activity that simply uses an everyday vegetable to test for acidity:
You can buy ordinary red cabbage at the grocery store, and use it as an indicator for acids or bases (instead of litmus/pH paper). The liquids (sodas) will change color according to their levels of acidity--a really cool reaction to watch! Here are instructions for how to create the red cabbage juice indicator:
As a warning: it's a good idea to supervise your child and use safety goggles when working with chemicals of any kind! Good luck!