A child in the North Lanarkshire area is receiving hospital treatment and a second has recovered. Both are aged under five.
NHS Lanarkshire said the youngsters visited Colzium Park in Kilsyth before falling ill.
A joint investigation with North Lanarkshire Council and Health Protection Scotland is under way.
Dr Josephine Pravinkumar, a public health medicine consultant, said: "We are currently 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 that both children visited Colzium Park in Kilsyth 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 on site.
NHS Lanarkshire has written to local GPs to make them aware of the cases. Symptoms include diarrhoea, stomach cramps and fever.
Dr Pravinkumar said: "E.coli O157 is a serious disease. It is very important that we are made aware of any further cases. I would especially advise parents, if their child has any symptoms, that they should report this urgently to their general practitioner.
"The disease can cause severe illness in young children and older people.
"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 that food is always properly prepared."