you could use your cell phone and an app called " Rocklogger". One of the subroutines of the Rocklogger actually turns your cell phone into a level and surveying compass.
HERES A QUICKIE TUTORIAL
1.Measure the distance from the one holding the cell phone to the place where you shoot off the Mentos/coke (Like NEAR A WALL would be best)
2. Measure the distance from the ground to the phone's eye level of whoever is taking the measurement. (that is the actual zero ground level for your surveying app)
3. when the mentos foam is shot off, someone has to measure the maximum angle from your eye level to the highest elevation the foam "fountain" achieves.
3.Use the trig functions in the Rocklogger app and(using a TAN function) solve for the right triangles elevation.(Then add the additional elevation of the ground to ye level of the measurer.
OR, if you have a hunter in the family. One who has a distance measuring range finder , you can accomplish the same as the app (except there would be a few extra steps)
Rocklogger is a student grade app that costs about a buck at the app store. It has a lot of other science and surveying tools on board so your kid can have some fun with it and HAVE EM BE OUTSIDE MORE.
In my mind Using any formulas that have "time components" require you to know how fast the mess is going and I don't think that's gonna work unless you know the vertical elevation in discrete units.