This probe also tests soil pH and light, but I don't think I'd count on if if I really needed an accurate pH. It has an easy-to-read meter, so I think it would be a fun project for a 4 or 5-year-old to tell dad or mom when it's time to water.
I ordered this "Active Air Three-Way Meter" from Smith & Hawken. (It's hard to believe they sell anything for $10!)
Got a favorite garden tool? Do tell.