NHS Greater Glasgow and Clyde said the "tame" fawn, which was hand-reared and vet-checked, was outside the Royal Hospital for Sick Children in Yorkhill, Glasgow, as part of a planned charity event in the run-up to Christmas, when a member of staff decided to take the animal inside.
It visited some of the wards to allow more youngsters see and pet it, but the health board said the reindeer had no contact with any patients in isolation.
NHSGGC described the incident as "well intentioned", but said the decision was taken without the permission or knowledge of managers or senior clinical staff.
In a statement, it said: "As such we conducted an internal review to learn lessons.
"At no time did the fawn urinate on a child and appropriate infection control procedures were taken with those children who petted the animal.
"The reindeer fawn did not go into any isolation cubicles or have contact with any patient in isolation."