I've read the Three Musketeers when I was 10. Here are some reasons I did not find the violence troubling, and how you can apply them to your writing. (And just to clarify, I wasn't a child who didn't mind violence. In fact, by age 14 I still couldn't watch big parts of X-men, for example, because the violence was overwhelming.)
First, you will not find gory descriptions in The Three Musketeers; no spilt guts, no blood spraying over everything (soaked shirts are present), no grown men screaming in pain (Aramis all but faints from the pain of a wound twice, and Athos makes his first introduction to the story collapsing in Treville's office, but their stoicism evokes respect rather than revulsion).
What do you take from it? Don't linger on the image of blood puddling on the floor, guards choking on their own blood, definitely don't describe how their bowels give out in the last throes of their suffering. Give them a clean death, and keep it clean.
Second, the violence is "fair": the opponents have a sword in hand, they may defend themselves, in fact usually they outnumber the heroes. Nobody is murdered in cold blood. If the heroes do not defend themselves to the utmost, they might find themselves dead on the battlefield.
This point you already have covered by the righteousness of your characters' goal (rescuing a friend), and by the fact that the guards are likely to respond with lethal force. Also, the guards are (I guess) stronger than the three teenagers. When violence is just, it's more like punishment. It's OK for the bad guy, and whoever happens to represent him in the scene, to get killed.
Third, surprisingly few of the Musketeers' opponents actually get killed. Many more are incapacitated, receive medical help, and later it is mentioned in passing that they recovered. Dumas was personally familiar with this aspect of the world he was writing about (duels were still being fought in his time), so I'm going to trust him on the realism of this.
It is up to you whether the guards need to be killed or merely incapacitated. I would guess that keeping them alive is not a priority for your characters, but the overall tone of the story would be softer if not all of them die.
Fourth, violence (duels) is a fact of life for the musketeers. They do not linger over having killed - rather, they rejoice at having survived and gained a victory. Other forms of violence are shocking, to the characters and the audience alike (Constance's poisoning), but that's done by the antagonist. Because violence is a fact of life, it receives less focus, and thus less time for the reader to actually experience and understand it.
I would imagine that in your medieval setting, some form of violence would be a fact of life for your characters. As such, the experience of "I've hurt someone" would be diluted for them, replaced with "I won". In essence, you'd be glossing over part of the violence.
When it comes to describing the scene, "Eddie stabbed him" is indeed blunt and bland. Instead, consider a tense combat scene - something that would draw the attention away from killing to the danger, and the awesome swashbuckling. Take a look at the duel scenes from The Three Musketeers (I only have the book in French, so can't insert a quote). Ivanhoe should also have something useful.