With the smaller-sized kiddie pools (the ones that hold perhaps 30 gallons or so), it is fairly easy to just empty out the pool every day and then spend a few minutes filling it up with fresh water every time you want to use it. With the larger-sized inflatables, however, this is considerably more of a time-consuming chore—and can make for some astronomical water bills, as the largest inflatable pool available holds over 700 gallons! If you'd prefer to just fill your kiddie pool and then let it stay filled for a while, you'll have to do a little maintenance on it to keep the water clean, just like you would with a "real" pool.
Skim the leaves and dead bugs out of your pool on a regular basis. You can use a pool net for this purpose, or even purchase an inexpensive butterfly-type net. Enlist your kids as pool skimmers and get a special net for each of them. With luck, they may even come to think of this as another fun game to play in the pool.
Look for a cover that will fit your pool. If you have a round pool that is 8, 10 or even 12 feet in diameter, you can use the same types of covers that are sold to fit the "quick set" type of portable pools. You can also purchase covers that will fit rectangular kiddie pools.
"Shock" your pool with a chlorine treatment. You can purchase this from a pool supply store, but the cheapest, easiest way is to use good old chlorine bleach. The correct amount to use comes out to about 1/4 tsp. of bleach for every 10 gallons of water, so if you have one of the larger-sized pools this comes out to 2 1/2 tsp. for every 100 gallons of water.
Drain your pool periodically despite using the above methods. This will get rid of all of the leaf and bug debris, dead skin cells and any other "accidents" that may have happened in the pool. If you have an exceptionally large pool, you may find that the easiest way to drain the water out is to deflate the pool, and this will make it easier for you to move it to a different location in your yard as well, should you choose to do so.