For maximum funabillity get a water flow controller on the hose. It just make it so much easier to use. I tested this setup at a birthday party and even the four year old managed to use it fine. 

@noelito have made a balloon tying tool! Check it out here.

https://www.printables.com/model/677867-balloon-tying-tool

This is modeled with the European standard. So all the threads are made with the ISO Pipe Threads provided by Fusion 360. I've made .stl models for quick connect, and all three tap connector diameters that I found.

I added the .f3d files for all of the models to make it easy to alter the model to fit what ever need you have.

The quick connect model needs a small O-ring to make a good seal. I just bought some spare O-rings made for quick connect plugs in the hard ware store.