My niece has a wet cough at night while sleeping and sometimes throughout the day. she doesn't seem sick though, what could cause this? she 5yrs old.

Cough: The most likely cause is nasal congestion leading to cough especially at night. Simple upper respiratory infections can cause this; so can allergies; so can over-the-counter over use of cough/cold preparations; so can other things. I'd take her to her doctor and ask if she/he might be able to recommend something simple to reduce the nasal congestion and see if things get better.