One in North Lanarkshire is receiving hospital treatment and a second has recovered. Both are aged under five. NHS Lanarkshire said the children visited Colzium Park in Kilsyth.
A joint investigation with North Lanarkshire Council and Health Protection Scotland is under way.
Dr Josephine Pravinkumar, a public health consultant, said: "We are investigating any potential common sources of exposure. People can become infected in a number of ways: through eating contaminated food, contact with infected animals or animal faeces, contact with other people who have the illness or drinking contaminated water.
"Our initial investigations have found both children visited Colzium Park shortly before becoming unwell. However, investigations into other possible sources are continuing."
The park has a children's play area, but does not have any farm animals and there are no catering facilities.
NHS Lanarkshire has written to local GPs to make them aware of the cases. Symptoms include diarrhoea, stomach cramps and fever.
Dr Pravinkumar said: "I would especially advise parents, if their child has any symptoms, that they should report this urgently to their general practitioner.
"The best protection against E-coli O157 is to always wash your hands, especially after contact with animals, after going to the toilet and immediately before eating and to make sure food is always properly prepared."
E coli 0157 is a bacterium that lives in the gut of animals, including cattle, sheep, deer and goats. It can also be carried by pets and wild birds. Simply carrying the bacterium will not normally cause an animal any harm or illness, but if contacted by humans, the toxins it produces can cause illness ranging from diarrhoea to kidney failure. In some case the illness can be fatal. Young children and the elderly at the greatest risk.