So, as a premise, as we look at our lives - as followers of Jesus, the results in our lives is not a dream of pretentious 'do's and don'ts', in essence it is not a dream at all, it is a real life that calls upon us to give of our all to become involved in the walk along with God's auspices in accomplishing His goals. So, the question, practically speaking is not "How do I get them do a thing" It is, "How do I allow them to understand the will of God about fighting?" Hmmmm..... That sounds harder....It is! For it is more time consuming and costs you more to administer parenting skills. But the good news is - this is a team effort. In fact, the Holy Spirit will take the lead if you let Him; working together to get His intentions and purposes accomplished; godly-counsel not withstanding.
So, first off - practically speaking - you must live a life of Christ centeredness yourself. Second, you must understand God goes before you and commissions you to speak and act toward the goal of His will on the issue of fighting. So research and pray as to what that will is. Thirdly, be active in your kids lives in other areas of life, such as school and friends and priorities of fellowship with like-minded Christian youths etc..in a non-militant loving capacity.
Listen actively, by asking good questions as to why they love to fight? And let them answer without pressure, accepting their point of views respectfully as the Holy Spirit directs you into how He would want you to give of yourself in helping them develop in the ways of the Lord. There are no magic bullets to this, just practical steps governed by the Holy Spirit. Good godly-counsel is also available so seek that out with the H.S. leading the way in that area as well. I will pray for you and your kids. May God richly bless you and your family.