Hi
Interesting question. The rejected water from the RO would be slightly higher in mineral solids than your water directly from your tap, but I wouldn't think there would be any problem on that side. What I would worry about is that you must have a chlorine removal filter before your membranes so your rejected water will be chlorine free - and the reason why chlorine is added to our water is to kill germs and bacteria.
I would probably advise against it, better to be safe than sorry - after all, how much will it cost to fill a small kids pool? But I would also say that the water in the pool must be emptied after every use as sunlight destroys the chlorine content added to the water, the reason why we had to add chlorine to our pool overseas every evening. Algae soon started growing in the pool, turning the water green, if there was insufficient chlorine in the water.
I used to direct the reject water onto the garden and the garden thrived, until the neighbours started to complain about their garden being flooded out, so it now goes down the drain.
Spruce