Assalamu Alaikum
You can first explain how the treatment of animals in Islam should be. Animals are living creatures and should be treated kindly and fed food which is suited for them. The environment they live in should also be suitable for them and not crowded and miserable. Fast food does not take into account the treatment of animals. They just want to produce food for large masses, in which case, animals are raised in bad conditions and their lives go to waste (eg. chickens are killed if they are male since male chickens can't lay eggs). In Islam food is not to be wasted so every sacrifice or animal that is slaughtered is taken into account and animals in general should not be killed unless there's a valid reason for it. Lastly you can explain the sunnah of slaughtering the animal such as how Allah's name is to be said on it, sharp knife to be used, and how it should not be slaughtered with other animals witnessing it, etc. Fast food is also in general unhealthy due to the chemicals and additives they add to it.
You can prepare beef patties, cook them, and freeze them and they can be reheated and used as "fast food" in the home for a quick meal.