So ask a doctor or better yet, a pharmacist. The fact that OTC “cold medicine” treats only the symptoms of the cold is not some deep dark doctor secret they whisper to you as they hand you your med school diploma. Now I don’t know exactly what “cold medicine” the OP is referring to, but if it’s something like NyQuil, I’m pretty sure there are: 1) zero documented cases of adverse reactions from not taking it, and 2) a reasonable chance that the medication is causing the child to sleep so soundly that she isn’t waking up from the urge to urinate.
If the kid is taking a prescription antibiotic, she should obviously not stop taking it without medical supervision, but that isn’t “cold medicine,” since colds are caused by viruses.
If you don’t believe me, perhaps the Mayo Clinic would be more credible in your eyes:
From the article:
"The Food and Drug Administration (FDA) and the American Academy of Pediatrics strongly recommend against giving OTC cough and cold medicines to children younger than age 2. Over-the-counter cough and cold medicines don’t effectively treat the underlying cause of a child’s cold, and won’t cure a child’s cold or make it go away any sooner. These medications also have potential side effects, including rapid heart rate and convulsions.
FDA experts are studying the safety of cough and cold medicines for children older than age 2. In the meantime, remember that cough and cold medicines won’t make a cold go away any sooner — and side effects are still possible."