There is a huge misconception that Game Boy is a platform for children, hence all the games must be safe for kids of all ages. This is far from true. Although there may be a plethora of family titles available, there are just as many geared to older children, teens and adults. You might pick one up starring a cartoon squirrel with the impression that it must be okay, only to discover that that the squirrel is a beer swilling womanizer. You might do your best to avoid games that sound violent to find the title is a code word for blowing puppies to gory bits. This is why the Entertainment Software Rating Board (ESRB) was formed.
The ESRB's mission is to rate game content based on the degree of violence, sexuality, language, etc. Although similar to the film ratings board, the ESRB rates on a different curve. You simply watch a movie, but a video game is interactive. If a character is shot, you're the one who did the shooting or the one who got shot. This is a difficult task that the ESRB takes very seriously. Not only do they require game content to be reviewed, but packaging and advertising as well.
To ensure you know the acceptable age range the ESRB created rating symbols and content descriptors. Rating symbols display what age the content is appropriate for, while content descriptors outline the questionable content.
Please understand these ratings are a guide to use in making decisions, but still use your best judgment on what you feel is most suitable for your kiddies.