Tyffani took her 5 and 3-year-old to the zoo. Within hours they were fighting for life.

A three-year-old girl has died and a five-year-old boy remains in a critical condition after a visit to a petting zoo took a fatal turn.

Kade and Kallan Maresh spent the day at a Minnesota zoo last week, where they happily posed for photos with baby animals. But according to their mother, Tyffani Maresh, within hours of their visit, the pair began vomiting and had “non-stop” bloodied diarrhoea.

"After several ER visits, we were rushed by ambulance to Children's at the U of M [University of Minnesota] on Wednesday when lab work discovered they both had acute kidney failure," Tyfanni wrote on the family's Caring Bridge page.

"The toxin from the bacteria not only damaged Kallan's kidneys but also her neurological system. Her brain and heart were being damaged. Our sweet sweet little girl lost the battle and went to heaven last night."

The cause of their illness was the deadly shiga toxin bacteria, which is behind viruses such as E. coli.

Kade is still in the hospital and fighting for survival.

"The toxins have not reached his neurological system but both kidneys. He has had a blood transfusion is on kidney dialysis and may need platelets. He has a long road to recovery and we hope and pray the toxins stay away from his brain and heart and other organ systems," Tyfanni wrote, asking people to pray for his recovery.

"It is absolutely heart breaking watching your babies in so much pain and being so helpless," she wrote.

But, according to her most recent post, Tyfanni says things are starting to look up, with Kade showing some "sass in his personality" on Tuesday, and even accepting a few bites of food.

"It's the little things that give us hope!" she said, adding finally, "Hug your babies tight and let your loved ones know you care."

While the cause of the bacteria is still unknown, the Star Tribune reports that the petting zoo animals have been taken "off display" as a precaution for other visitors.