Most OTC cough and cold meds for young children don't do much. The only one I have found to be helpful is Delsym...it is great and you can use for kids and adults. Here is info/dosing on it.
http://www.askdrsears.com/html/8/T089701.asp
BUT....before you give her anything, I think you need to talk to a doctor again. If she still has croup, a cough med is not going to help. Croup inflames the air sacs and air passages and cough meds aren't going to help that....only steroids and breathing treatments. Also, if they said she had an upper respiratory infection, she should have gotten an antibiotic for that. If it has been over 3-4 days and still has the same symptoms I would ask about that....esp with the green mucous. In addition to an antibiotic, if she doesn't have croup anymore, I would get clearance about giving her the cough med at night since she is a little young and then just give her ibuprofen alternating with tylenol for the fever when it gets high (usually I do over 101) and seems to be bothering her. If it isn't bothering her, it is better to just let it run it's course cause those meds give them a false sense of wellness and then they run around like they aren't sick and exhaust themselves.
If it is croup, cold night air (we got plenty'o that!) helps with an attack, or sitting in a steamy bathroom.