Apparently approved rat poison comes in small open-ended boxes which larger animals can't get into.

But there still may be a problem of cats and dogs picking up dead or dying rodents and ingesting the poison that way.

It seems likely that the poisoning of pets is happening accidentally, but there are some disturbing forums in which people say they leave poison out because they object to neighbours' cats coming into their gardens, and feel that this is the only way to stop them. This is illegal and carries quite heavy penalties.