I did similarly when I was a kid. When I had my own family, I remembered how I wished, as a kid, I'd had rules for fighting large battles with my Army Men figures.
My sons loved my 28mm fantasy miniatures, and of course they wanted to play with them, but having paid handsomely to have a large number of them professionally painted, I could not allow that. I saw that my sons had plastic Army Men in their toy collections, so I hit upon the idea of writing a set of basic, introductory war game rules they could enjoy with me.
I later printed copies at my local office supply store, using their photocopiers, selling them through a free website. I ran local Community Education classes for the public, introducing them to war gaming with plastic Army Men figures. It was not terribly popular with the adults, but my youth classes always sold out, with the School System asking me to run additional classes to meet the demand for them. I sold a number of my rules sets through the classes, but the majority were sold through mail order from the website. I ran the website from 1998-2007, before I retired it.
I still play Plastic Wars, with two of my three sons, when I can (my third son lives half a country away, in the deep South of the USA, while I live in the North). I've updated the rules a few times, hoping to re-publish them maybe next year, in a PDF/POD format: loads of scenarios, loads of color photos, loads of instructions on how to build terrain, how to paint and base the figures, etc. The rules play fast and fun, the cost to get into the game, is low. The rules work for small games, up to very large games, with ease, as long as you have enough players to move all of the figures in play. The largest game I ever played was on a 12' x 30' table, and it was a blast! We actually played two different, but similar games on that table setup.
The linked photos are of the first game; for the second game, we added a mountain with a train tunnel, and hydro-generators along a river. The train was delivering reinforcements and supplies for the Tan Defenders, a few Turns into the game. The Green Invaders made a beach landing, slogging their way towards the town. The Tan's had a sniper in a water tower who killed more than a few Green's before he was neutralized. When the Train arrived, the Green's hurled what they had at it, destroying its cargo, killing more than a few of the relief troops. In the end, the Green's scored a pyrrhic victory, paying a very dear price for it. Those two games were more fun than a Human Being ought to be allowed… Cheers!