Personally, having built long term ponds (a bog, minus the dirt), and "bog filters", I would not use a kiddie pool as a liner unless you want this to be temporary or plan on moving or redoing it in a year or so. My personal preference would be to use a layer of sand, a barrier (threaded plastic tarp will do), and then EPDM. That, or I'd use a prefab pond. Kold Ster-il, RO, rainwater, buy water, it's not going to fall into your lap (unless it's raining). Pick your poison on that end. Right now, I go through 1½ gallons/week with a terrarium, a Nepenthes, and an odd CP here and there, that includes misting, and misting my 30some bonsai (the bonsai get watered with tap water). A bog in the sun will gor through substantially more. I'd look into a water purification system, JMO.