Oxalic Acid is a naturally occurring compound, but it is fairly poisonous in its acid form. The best way to deal with it after you are done using it, is to add baking soda to the kiddy pool and stir. Check the pH of the solution with a pool testing strip and if it is higher than 7, all the acid would have converted to its Nitrous salt form that is safe and harmless. Feel free to dump it on your lawn at this point, or if it is winter (and I just noticed that you are in SoCal) on your driveway as a safer ice melting alternative (better life through chemistry...)

Here is some (probably more that you want to know) info:

http://www.1911encyclopedia.org/Oxalic_Acid