"how do you think my dwarf caiman feels with it."
He will have a nice, dry, toasty basking spot. I am sure he will be fine with it. A good portion of the alligators at St. Augustine alligator farm appear to be on sand. I dont think they ever have any problems.
"and do you think it could be a problem if he eats some sand when feeding, just a litle beat, i feed him over a plate, but if food goes down and he pick it up again with some sand he will eat some"
If he eats a little bit it shouldnt phase him at all. Crocs get dirt and all kinds of stuff all over their food many times. Doesn't appear to hurt them at all. Crocodilians also ingest stones pretty frequently, and this actually works to their digestive benefit.
Also
"as dwarf caiman defecates in land, I think play sand could work like "catsand", and poop cuold be scooped out
opinion? "
Sounds like a plan to me!!