I too thought this must be an urban myth, until I gave it a go (3am sleep deprivation desperation). As soon as I did this, my toddler slept and stopped coughing every 2 minutes. However, it was combined with a humidifier in her room, a dose of calpol, and sheer exhaustion. I now do it as a matter of course whenever she falls ill, and it seems to at least take the edge off. It's worth a try, in my experience. Good luck, I hope your dc is better soon.