From our experience we have found that children and pets do not eat rubber mulch or have the urge to eat rubber mulch similar to how they do not eat wood mulch.
The Maryland Environmental Services commissioned laboratory testing on the effects of stomach acid, via hydrochloric acid, on 3/4 inch tire nuggets. "Visual examination of insoluble residue appeared to indicate only fibrous reinforcing strands were dissolved by the hydrochloric acid. The tire rubber did not appear to be affected in any way; i.e., chalking, cracking, fracturing, etc." (PSI Report No. 486-40013-001)
If a pet or kid accidentally eats a rubber mulch nugget, the nugget should pass through like any indigestible material, without acute or chronic issues. An upset stomach may occur depending if more than a single nugget is swallowed.