Can my unborn baby catch a virus or disease in the womb if I work in a children's hospital?

Depends. If your immunizations are up to date the major risk to pregnant women in hospital would likely be cmv. To acquire this one would have to have contact with blood or other bodily secretions intimately. You didn't say what you did in hospital or how close you are to patients, and if close, what types of patients. The risk is probably very very small.