Yes, you should not use Cocoa Shell mulch if you have a dog. Many dogs find the taste of Cocoa Shell irresistible and will eat it. The trouble is that Cocoa Shell contains high levels of theobromine which dogs break down slower than humans, who have a higher toleance. Consumption of large quantities of cocoa related products by dogs can cause severe gastro-intestinal symptoms, epilepsy and heart attacks which may prove to be fatal.  The product is not appetising to cats and therefore is is very unlikely they will consume harmful amounts of the material.