Freezed water in the carburator starts to be a problem even a few degrees above zero (celcius), because of some vaporizing process taking place, reducing the the temperature in the carburator.
Days with pourring rain or a lot of moist in the air also causes problems, that these kinds of products can reduce, since the water in the carburator is most often caused by moist from the air-intake. So it's a very good idea to use this from +5 to -1 degrees celcius. Below that there is not much moist in the air, and you won't experience the problems as much.
I am still more and more sure, this will help you out. Last thing you need to work out is what those kinds of products are called in the country you live in. I think your father can help you out with that, else ask a scooter dealer or ask at a gas station.