My parents (mostly my mom) felt similar when I first asked for all four Assassin's Creeds at the time my freshmen year for the holidays. I ended up getting them, and I can easily say that those four certainly are more Teen than anything. Assassin's Creed III wasn't bad at all, but was a little more rough in the violence category at certain parts in the game.
Black Flag is probably more "mature" than Unity in terms of suggestive themes and other elements and probably is the most "mature" out of all the Creeds. Unity barely has enough story for you to notice anything. I can't speak for all of the Murder Mysteries or Paris Stories in Unity because I haven't done all of them, but Unity is definitely cleaner than Black Flag in language and suggestive elements. But Unity definitely has more gore. It's really only in the cutscenes though and one of the cutscenes in Co-op, all of which you can skip. The only other gore present is sometimes there are people's heads on pikes in various mons throughout the environment, but it isn't too detailed and you'll be too busy running everywhere to notice.
As far as the violence goes, you can turn off the blood in the options menu. Compared to GTA V, both games are Teen at best, in my opinion.
For example, Okami is rated T for Teen here but has little to no blood and is extremely stylistic, and probably has more suggestiveness than Black Flag or Unity combined.
If you were allowed to only play one, Black Flag is definitely the better game. While the physical assassin element arrived late in the game, you witness the developement of one of the greatest casts in the history of the Assassin's Creed series, assassins or not. The ending is one of the best. The gameplay certainly favors the pirate element, but the story, both in the present-day storyline and assassin storyline, is one of the best and most well-balanced. While it isn't as rich in history compared to Assassin's Creed III and the preceding Creeds, trust me when I say that the story makes it worth it and I guarantee that you will even learn a few lessons from it.
Unity has a lot of history to offer, but sadly a lot of the main mission/main storyline elements didn't center around many historical events during the French Revolution as it could have. A lot of these historical elements were instead divided between singleplayer and Co-op, which I felt that the singleplayer storyline ultimately suffered because of it. It'll keep you busy, but it is far from the best game in the series.
Lastly, read the reviews for both games on commonsensemedia.org . If that isn't the right address, just google it. I used to do that when my parents acted the same way when I was younger. They should all be very positive and for the most part supportive of teen (and even younger) play. Hope that helps.
YOU SAY YOU WANT TO GET BETTER AND YOU DON'T KNOW HOW.