Part of a water park has been closed after dozens of visitors said their children had been “violently sick”.
Public Health England is investigating the reports which centre on the water play area at Twinlakes in Melton Mowbray, Leicestershire.
But parents have been reporting their children feeling unwell since going.
Public Health England said it was investigating and a water play area had been closed as a precaution.
Lucy Marriott, from Nottingham, took her children to Twinlakes on Wednesday as part of a school trip.
She said her 12-year-old daughter and seven-year-old son became unwell early on Friday.
They were both “violently sick” and her son was “doubled over in pain”, she said.
Kelly Quinn from Melton Mowbray also took her children to the Twinlakes water park on Wednesday.
Connor, eight, and Cara, 10, have both had “severe diarrhoea and projectile vomiting” since Thursday.
Ms Quinn does not believe heatstroke caused the illness as the family took precautions by sitting under a parasol and having “plenty of drinks”.
Both children have been sent a sample kit to try and find out the cause of the illness.
Another mother from Melton Mowbray, Cara North, was concerned by the cleanliness of the theme park during their visit and said areas looked “absolutely disgusting”.
She said she was so concerned about her daughters she took them to the doctors, where they were given rehydration drink sachets.
Public Health England said it was working with the Health and Safety Executive, Melton Borough Council and park bosses “to investigate the reports and ensure that any necessary public health actions are taken”.
It added: “As a precautionary measure, the water play area will be closed until further notice.”
Twinlakes has been approached for a comment.