Dear Paula,
There’s no data at this time indicating that cranberries (Vaccinium spp.) are toxic to dogs—or any other animal species. However, the berries can cause gastrointestinal upset, which can result from the ingestion of any plant material.
Nonetheless, preventing urinary tract infections with cranberry juice is a topic that should be addressed with the dogs' veterinarian. If the dogs are getting frequent infections, their vet is the best expert to help find an appropriate therapy.