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.
John's Wargame Page 
Excellent solution.
One little suggestion only for the beginners set:
I would make inf. firing more difficult (classic, stimulating charges) than inf. melee.
So: inf. firing: needing a 6
inf. melee: needing a 5 or 6
cav. melee: needing a 4 to 6.
art firing: as you described.
Saves as usual.
It seems to me that your rules can not only be used for horse and musket period but are excellent for every period. Changing muskets by bows, rifles, ….
Nap: less saves (cover), medieval/ancient: cover AND armour saves.
Modern period: more number of dice for heavy weapons (tanks…).
My 7 years grandson is – like me – already enthusiastic.
Curious for your next step.
Cheers,
Lex
Next step is to actually write them of course! 😀 Might take a week or two. Your suggestion is a sound one. I like it! So infantry hit on a 6 at range and a 5+ in melee. BTW, my last post has enough information to actually play a game.
I am working on some paper figures so I can test the rules quickly. Painting can take such a long time as you well know! So I will probably have the basic and advanced rules drafted in a couple of weeks. I’ll add optional rules as they come up.
Probably the next set will be WWII. I think a skirmish style game works best for that period. I also plan on ancients as well as skirmish sets for both ancient and horse and musket.