How toxic is a mouthful of nasty-tasting detergent?
Probably not too horrible if the child’s mouth can be rinsed out fairly soon (assuming the experience doesn’t send him to the faucet quickly to do just that himself). Some minor chemical burns and irritation from the sodium carbonate and enzymes. It’s unlikely to have lasting ill effects.
“Keep out of reach of children” is doubtless on these packages.
Unpleasant? Sure. Toxic? Probably not, especially if the tyke is smart enough to spit it out when it tastes that bad. He might have a severe case of the poops for a day or two but it likely will not kill him.