Five nurses have been suspended from the children's ward at Scunthorpe General Hospital, the BBC understands.
Northern Lincolnshire and Goole Hospitals NHS Trust started an inquiry after allegations were made that a baby was restrained on Disney Ward.
Humberside Police confirmed they were also investigating the incident.
Wendy Booth, the trust's head of governance, said: "We can confirm an incident has occurred which the trust is investigating."
She added: "We are keeping the family fully informed. It would not therefore be appropriate to comment further at this stage."
Disney Ward caters for patients aged from a few days up to 16 years, who have a variety of medical and surgical needs.