I suggested grit as they are just on a patch of grass not really running round where they will find grit sized rocks. Having had compacted crop surgery here full of wound up undigested grass I like to err on the side of caution these days when they start eating grass and make sure they have something in their crop to grind down that grass. You can buy stone grit for birds at the pet shop, I was surprised just how large the stone size is for just a cockatiel, nowhere near plain dirt size or sand. It wasn't unlike the finer gravel size of fish tank gravel but not as smooth. So unless they have access to fine stones I'd be giving them bought grit.
A couple of years ago I saved a cockatiel from crows. Sounds like I should give it grit. They're not really ground foragers.