Personal Opinion:
Teach your 10-yr-old some math. It's finally where you can say, "Look! I can figure out how high this rocket is going to go by using math!" Stick a protractor on a piece of wood with a sighting straw and a string w/ a weight on it. There's your home-made altitrack.
Forgive me, I'm not making any assumptions here. What you are doing is measuring a certain distance from the launch pad, and then when the rocket is launched, measuring the angle that the string indicates when the rocket reaches its highest point.
The formula you use to calculate the height is:
tangent(angle) = distance to launch pad / vertical dist. travelled
-or-
vertical distance = tangent(angle) X distance to launch pad
Once you have the angle, you can use a calculator to find the tangent value, multiply by distance to launch pad, and there you go!
This can be used to measure the height of your house, a flag pole, or a tall building as well. PLEASE! Do this as a favor to your child.
A homeschooling dad.