Water Bottle Rocket Launch
If you're struggling a bit and need more help, there are water bottle rocket kits you can buy that have all of the materials and step by step instructions on how to build rockets. I ran across one in a toy store the other day.
I can't attest to the quality of this kit and I can't say whether the kit rocket launches higher than the homemade rocket, but kits are an option. This particular kit was priced at $9.99 and they had others going up to $19.99. I'm guessing the more expensive models launch higher. This is potentially a good gift idea for the budding scientist in your family!