gets bad (it's constant, or it sounds weird, or your kid is looking tired, etc.), even if it's just started, immediately get to the doctor.

One of my kids got pneumonia at 6 months old. He had a cough, I took him to the doctor, she said wait and see. I took him back three days later, the doctor on duty said he was too cheerful to be very bad off, but then while examining him discovered all kinds of badness in his ears and throat, and a horrible sound deep in his lungs (the doctor said, "I've never seen a child so sick present as so happy"), and sent him to the ER for x-rays, which confirmed pneumonia. The first round of antibiotics (an injection) didn't wipe out the mess, so he was given a stronger one, and then a stronger one, which finally did the trick.

For a few years after that, every time he got a cough, it'd go bad, fast. I had to bring him in for breathing treatments so often, they decided to prescribe him a nebulizer and albuterol, which I was instructed to use on him as soon as cough symptoms started.

For my other kids, they'd get a cough, it'd go away in a few days with fluids, rest, and maybe some over-the-counter meds.

So, this has been my experience. I'd say don't hesitate to call the nurse line if you're worried.