Now, is it "sandy" or kinda "flakey sandy". When I worked in daycare there was a young biy(2ish) who regularily had flaky gritty poops. Really hard to clean! Anyways...it turned out that he generally ate a lot of oat cereals in the a.m..
If eating sand isn't the culprit, it might be something in here diet. Not an allergy perse, just the way a particular food gets processed in larger quantities.