I think Rafael explained it very well, I just would like to add, that you probably confused these two words because they are pronounced in a very similar way. Also Brazilian native speakers say and write it wrongly, although the meanings are quite different. So, you should pay attention, even when Brazilian people talk to you, because they may be making this mistake.

Hi, Ramon! The word "a" in Portuguese can be an article or a preposition. In this case, it is a preposition, that's why it is not the same as saying "The father and the son are watching the a movie". The word "a" is needed because of the verb "assistir", which requires this preposition when meaning "watch". If you didn't use this preposition after this verb, the sentence would have a whole different meaning, once that "assistir" would be the same as "help". In Portuguese, some verbs can mean different things depending on the following preposition, you can get to know more about it by studying "regência verbal". I am also on your disposal to explain any doubts you might have.

I don't know in which level my German is, even though, we can practice it, if you would like to! I have already lived in Germany, so I think that I would be B1 or something like it.