Here is something important to think about, osmosis.

The water in the pool is more saline than your body chemistry because of the chemicals, and even though you stay in the water on a sunny day - you will become dehydrated as your bodies moisture is leeched out through the cell walls. I spent many years as a lifeguard watching this happen over and over to people.

I would iimagine it is the same with a snake, and since they're much smaller it would take a greater toll on them. The larger snakes though I think would be allright for short periods of time.

Ingesting pool water could affect it's kidney functions though it's eyes would be protected by the eye caps.

The kiddie pool idea sounds good though, fill it and let it stand a while to equalize with the ambient temp and put something in it for them to crawl out onto if they get tired.