As a small boy (a long time ago) I had a water rocket toy.
I decided to build one for my grandchildren. The base idea that the toy will be (relatively) safe so that young children can play with it. I do not have the ambition to set height records etc.

You can find many design on the internet, also some based on a Gardena type garden hose coupler.

You can also find an adapter for the bottle, gardena faucet part and rocket fins here.

This here is my design.

Fort the launcher platform:

Parts to buy:
1) A Gardena garden hose coupler
2) A piece of pvc tubing fitting the Gardena coupler
3) A few meters of smaller pvc tubing approx 0.25 inch to be able to pump the rocket at a safe distance. With a special Gardena coupler you can connect directly to 12od pvc tubing.
4) An air valve. I used a car type from an old bicycle tube (fitting in the 0.25 inch pvc tube)
5) Tube clamps for the tubing you use.
6) An 8 mm wooden rod for the trigger handle to be cut to size. 
7) 4 camping type thin ground pins (max 5mm). To secure the launcher to the ground.
8) A piece of rope (max 3 mm thick) to release the rocket from a safe distance.
9) A M3 fastener and nut approx 10 mm. You have to cut it afterwards.

Parts to be printed:
1) The launcher. Print it with 0.3mm, 10% Infill and supports.
2) The clamping ring around the Gardena coupler. Print it 0.1mm 30% infill and with supports
3) The trigger ring. Print it with 0.1 mm, 30% infill and supports.

Please be carefull! Anything under pressure can explode. Also objects falling down can hit someone!

I do not take any reponsibility or liability for accidents or damages