Studying the etymology of words can help with understanding their meaning. What I like about studying the 4 most common prefixes is that they make up 97% of prefixed words. These prefixes once understood, can help begin to make sense of an unknown word.
The three most common prefixes are dis, re, and un. Re means again and dis and un mean not. The fourth actually has four different versions. They are in, im, il, and ir. They also all mean not. If you are serious about increasing your child’s vocabulary, then whenever you see these in a book point out re, dis, un, and in. These will start getting your child into the habit of looking at the word and scientifically diagnosing it to see if there is anything that he/she knows about the word to understand it’s meaning better. This will in turn ensure that your child will become a better reader and increase reading comprehension.
If you happen to know the meaning of other prefixes and want to bring that to the attention of your child, feel free to do so. I am all about getting the most bang for you buck. However, it is not necessary. With all of the other tools that you are learning here we want to somehow manage to keep it all simple. Once we make reading difficult for the child, they will begin to disengage with us or possibly the text.
Another thing to consider is the age of your child. Obviously a high school student would be able to get deeper into analyzing word part meanings more so than a first grade student. So take your audience into consideration and enjoy this one on one time with your child.