As I said elsewhere, you can drown in a few inches of water if you are unconscious. Same is true of other liquids: if you cannot keep your airways clear you'll drown. You won't sink in, though, unless you fall from a significant height.

The real problem with slurry pits is that they are deep and quite difficult to get out of once you fall inside. One will also most likely be fully submerged when one falls in and has to swim to the surface. A person - especially a child - could wear themself out in a fairly short amount of time by panicking and trying to get out or get help.

The point I'm making is that you won't be sucked in. You can easily swim in the slurry. Hell, you could float on your back and wait to be rescued. (Though floting in a liquid mixture of cow shit and god knows what really is very low on my to-do list.)

Drowning in grain depends somewhat on the definition of drowning. (The one I'm using is death from asphyxia due to suffocation caused by water entering the lungs and preventing the absorption of oxygen leading to cerebral hypoxia, to put it very wordily.) Can you asphyxiate inside a grain elevator because you get engulfed by the grain? Most certainly. Is that drowning? I wouldn't call it that, but that's semantics for you.

Grain - or any granular material - behaves differently from fluids in certain respects. Flowing grain can have a fairly low density and that makes sinking in and asphyxiating a possibility.