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.

After looking around on the internet, I found designs based on the Gardena garden hose coupler.
My design is also based on this Gardena coupling and on a 0.5 litre PET mineral water bottle. However this adapter can be used for 0.5 - 1.5 liter bottles. That is why I have uploaded 2 fin designs.

The idea is the the air pressure pushes out water in the rocket so providing upward thrust.

To make the rocket stay vertically as much as possible, your center of gravity must be higher (approx 1 diameter of the rocket) then the center of pressure. That is why I made the fins go below the nozzle.

The design details of the rocket iself I leave to you. You can find many ideas etc. in the internet (search for water rocket)

You can find my launcher platform design based on the Gardena coupler here also, or in the internet.

To build this you need to print the adapter, and the fins (3X).

Print the adapter with 0.1 mm layers and 100% infill. The fins can be 0.2 mm with 15% infill. You likely have to sand the fins and/or the fin mountings a bit. The tolerance is tight. Also the adapter needs a bit of polishing and a bit of silicone grease to help make the adapter airtight.

You have to buy the Gardena faucet part and a rubber gasket.

Of course also youhave to "organize" a soda bottle".

I myself did not go above 40 PSI pressure. 

Be aware you are on your own. Be carefull. Anything under pressure can explode!

I do not take any responsibility for exploding bottles or rockets hitting persons.