Grapefruit Seed Extract, while it is very bitter it is very safe. This will kill candida and also infections. It would be safe to give your child 3x per (according to package instructions) day for 7 days during an infection. Or there is also oregano oil which is a great antifungal and antibiotic. (I'm not a doctor, but am a mom and this is what I would personally do)
The real trick is figuring out if the child has a virus or infection. Tell me something, do you know how your doctor diagnosed? How did he know for sure that your 2.5 yr old had a sinus infection? You know your child better than anyone.
The other thing you can do is take your child to the doctor, get the diagnosis, throw the prescription in the garbage and go to the health food store and get an alternative.