if a toddler can eat barley they can certainly swim in a pond that has a little barley residue. assuming the pond doesn't have any other hazards or contaminates or even snapping turtles. and assuming you watch him like a hawk when he is swimming i see no problems with the barley.
can a celiac toddler swim in a large pond treated with barley straw for algae control? . our two year old grandson will be visiting our home soon and we hope to let him swim in our swimming/fish pond that has been treated with a few small bales of barley straw to control the algae.