A poster at the Catholic Answer forums is concerned about her boys' interest in medieval warfare. The only concern I would have is if her boys want to be the princess and not the knight rescuing the princess.
My son and almost every other boy I know is genetically disposed to making weapons. This morning my son picked up a piece of Brio track with a gatehouse attached and said he had a "house gun". He also makes guns out of sticks, bread, rocks, lego and flashlights. One of his favorite toys is a Nerf bow and arrow. We didn't raise him this way. We didn't say "Andrew doesn't have any guns, let's show him how to make them out of sticks." Nope, he did this all by himself.
My favorite toys were little green army men and the castle Lego sets. I was enthralled by chivalry, knights, jousting and military tactics. I spent hours reading about D-Day, Custer's Last Stand and the Crusades. The intricacies of battles, the great failures and the courageous leaders filled my days and helped me form an idea of what a "man" was.
He was brave. He protected the innocent. He told the truth. He cared for his soldiers. He defended his family. He was chivalrous. While chivalry has fallen out of favor thanks to feminism, women have gotten the raw end of the deal. I have yet to figure out how it is better for men to treat women like men instead of like women. Men have always been able to do horrible things to each other because they were men. Codes of chivalry and "being a gentleman" were devised by MEN to keep the innocent (women and children) safe from the brutality and roughness of men.
Our culture today could use a good dose of chivalry. Some organizations, such as the Boy Scouts, still maintain remnants of chivalry in their codes. For a quick introduction to chivalry and some ancient codes we recommend the Baronage Magazine. For a look at how to bring chivalry back into our culture, I recommend the Compleat Gentleman.
But, like anything you want to teach, you have to start with children and you have to guide them through what they read, watch and listen to. If your children are in a public school, good luck. I can't tell you how often I am thankful that my wife homeschools.
I think one of the biggest draws for boys to war and war stories is a clear definition of "good" and "bad" guys. The falcon lego soldiers were the bad guys. The lion soldiers were the good guys. This is a point made by Michael O'Brien's A Landscape With Dragons - Children need to read stories where there is a clear distinction between good and evil. This is why series such as the Chronicles of Narnia, Chronicles of Prydain and the Lord of the Rings are much better literature than series such as Harry Potter and Dark Materials. It is clear which side is good and which side is bad. Good guys may have flaws and do bad things - Edmund in Narnia, Boromir in LOTR - but these things are recognized as bad. Until you have clearly established with your children what "bad" and "evil" are, you should not introduce literature that muddles the issue.
What kind of shows and movies do your kids watch? When they watch something are you paying attention to the message in the show or just its entertainment value? My dad started a movie night for teen boys and their dads where they get together every few weeks and watch a movie (usually a "Clean Films" version) and then discuss the virtues presented in the movie. This week they are watching Master and Commander.
What are your kids listening to? It may be popular, but it probably isn't moral. Take your typical "safe for the family" music station. The djs may be "family friendly" but what they play usually isn't. Think about "Follow Me" by Uncle Cracker or "Why Does it have to be Wrong or Right?" by Restless Heart. This stuff is telling your kids that cheating on a spouse is okay. Think Country music is any better? At least a third of the songs on country radio are ones I wouldn't let my kids listen to because of the lyrics. Toby Keith may be one of the top country artists right now and has become an icon for patriotism but his videos and music (Who's Your Daddy, Stays in Mexico, etc.) are basically Cowboy Porn.
Raising boys is tough but one worry shouldn't be that they like weapons and war. Use the interest as a lead in to the idea of chivalry and being a gentleman. Then, go buy them a catapult!