Dude you got to remember that your going to have to clean up their ****. If i was you i would go with a tile thats easy to clean up and still looks good. A sand pit sounds ideal but that would be some good maintence. They would get sand all in the ponds and ruin pumps etc. The **** would get piled under it, i just see sand making a big mess. I would do tile or some other surface of the such that i could just take a hose to it. Or a soil/mud with worms and other micros to break down waste, with heavey plant life.
UPDATES ALREADY LP WTF!?
lmao crocs and fish > srt6