I’d suggest you have a look at how other people deal with the mechanics of their LARP games. That way you can check out how these things are done already and add your ideas on top of this.
Several people have systems they will share with you, providing you acknowledge their work should you “borrow” any of it. As for Kids larps, well, kids are natural role players, you don’t need many rules. I have played many a simple combat game with my kids out on the street with only this rule - 3 strikes (torso/limbs only) and your dead/unconscious for 20 seconds.
I have a variant, where I play a zombie, the kids have 1 HP and I as the bad guy have as many HP as there are opponents, which also works quite well.
As a side note, I have played with weapon looting rules, where kids may take the weapon of their defeated opponent as long as they leave their own in place. Typically this means the sneaky kid will actually backstab his team mates to get a bigger weapon or shield.
Though not intended for children, the Knightshade Realms and Knight-Fight games systems I have posted in the Knightshade thread may prove insightful to you, even if they are not directly applicable.