Instead of the kiddy pools, I got the shallow pans that are sold to put under home water heaters. Any big box store hardware store will sell them. Just make sure to get the ones without the drain holes already added. You can put 3 to 4 fair sized pots in a single pan. You can also buy the larger shallow pans that are designed to go under washing machines, but they are more expensive (I am not sure how well the plastic will hold up to the sun).
I also tried the re-usable grocery bag idea, as a cheap replacement for buying grow-bags or root-pouches. The grocery bags seem start breaking down after a few months. Some brands more than others. I think the sun is the biggest reason, along with constant moistener. I will be moving all the trees that I tried this with into real grow-bags this coming spring.
CliffH