I don't know if all reptiles carry salmonella, but I do know they can transmit fewer diseases and cooties to us than the typical family dog or cat.
Check out
Zoonoses- by Daniel S. Shapiro, M.D.
That should give you a bit of ammunition.
Be sure to compare not only the number of diseases, parasites and health problems between reptiles and the common family pets, but the severity of them.
Oh... and rub in the fact that dogs, cats, and even 'food grade' animals like cows and pigs carry salmonella... not to mention chickens and eggs. And the innocent looking rodents that they keep buying for their kids can pass it on too.
If they want to know who Daniel S. Shapiro, M.D. might be and if he's to be believed:
Director, Clinical Microbiology Laboratories
Boston Medical Center
Boston, Massachusetts
Assistant Professor of Medicine, Pathology and Laboratory Medicine
Boston University School of Medicine
Boston, Massachusetts
That should give them something to think about for a while.