Personally, I would NOT use cocoa mulch if there was any chance that a dog might have access to it. One or two balls eaten can cause serious problems for small dogs, and by serious I mean organ failure or death. Even birds can get sick from eating mothballs or mothball flakes. I do endorse using iron phosphate slug baits, because they are not only effective, they are less harmful to animals, PLUS they do not pollute the environment. These products are a major cause of emergency trips to the vet, or even death.
Fertilizers containing iron have the potential to cause problems, but very infrequently.
Usually a dog must eat a LOT of an iron-containing fertilizer in order to get sick. Cats don't normally eat fertilizer, as a rule. Weed 'n feeds not much of a worry, either, although extra care needs to be taken to observe if the dog eats a large amount. When it comes to organic fertilizers, again, there are almost no health problems posed. However, because of the nature of organics, there may sometimes be gastrointestional reactions like vomitting.