I had a galvanized stock tank set up in my driveway for the kids, for years. I dumped in a "glug" of Clorox every day. Given all the splashing and having the hose in it nearly all the time to refill and play with that water stayed amazingly clean. If it got too muddy I would just empty the tank and refill it.

It's a very simple kiddie pool, easy to maintain and tons of fun for the kids.