How can one judge the ritual impurity (najāsa) of swimming pool water that has changed color, if there is uncertainty whether the cause of the color change is urine, given the presence of many children in the pool?

If water changes, and there is doubt about the cause of the change—whether it is pure or impure—then the fundamental principle is purity. It is not to be judged impure unless it is confirmed that what changed it was impure. This is based on the hadith: "Leave that which makes you doubt for that which does not make you doubt." The change might also be due to stagnation. So, if there is doubt about the impurity of something mixed with the water, the water remains pure.