Great Build.
Sand shouldn't be an issue, the gravel is much more dangerous. Larger pieces of gravel will get caught in their throats/digestive tracts and impact the bowels. Sand passes through much easier. I'd think that it'd be best to not feed them food that sinks to the bottom, I still think the sand won't hurt, but why tempt fate?
As far as the vet goes, you're probably going to pay a pretty penny. "Emergency" visits for sick cats and dogs usually will run you at least 200 or so bucks just to get seen, not to count any tests, lab work, medicine, etc. that you'll incur. Turtle vets are clearly more specialized and rare (read: expensive and overpriced), so I'd try any number of remedies and online help before taking them to a vet. Best of luck with it all!