I like comments. They keep me on task. Lex, in the previous post, rightly points out that if this is a kids game, maybe things should be even more simple. The rules shall be broken into 3 parts: Basic Rules, Advanced Rules and Optional Rules.
The basic rules will be very bare bones. infantry will have 12 figures, cavalry 8 figures and artillery 4 figures. There will be no formations. Infantry and cavalry fight in line. Cavalry move 10″. Infantry move 6″. Artillery move 4″. You may move any direction and maintain your facing or you may change your facing and not move. All turns are around the center of the unit. Infantry shoot 1 foot and hit on a 5+ (melee and shooting). Cavalry may not shoot but melee with a 4+. Artillery have a 48″ range. They hit on a 6+. At 12″ or closer they hit on a 5+. All units get a saving throw. Infantry and cavalry need a 6+. Artillery need a 5+. A +1 will be granted to infantry and artillery for being in cover (behind a fence/hedge/earthwork). Morale checks are done when a unit (Infantry or cavalry) reach 50%. The unit checks needing a 4+ to stay. Otherwise the unit is eliminated. The unit is also eliminated when it only has 3 figures left for infantry or 2 figures for cavalry. Artillery fight to the end.
The advanced rules will add slightly more detailed rules for morale, range effects, movement, different types of terrain and so forth.
Optional rules will add generals, unit characteristics (grades), unit officers, standard bearers and so forth.