Interestingly, pyrethrum derived from chrysanthemums and used as insect repellent was the original basis of the U.S.-China trade in the 1800s. DEET works for the latter batch of critters, but seems worthless for sand gnats, at least in the form of Deep Woods Off or Cutter. Anything can be dangerous if used inappropriately. However, if you're worried about it, his recommendation is a safe one for infants, if erring a bit on the conservative side. I'm not sure I'm going to let my 1-month old suffer if more conservative protection measures prove inadequate.
Cost is the only real issue.
Treatment of mosquito bites with cetirizine. It works well for my wife who urticates dramatically and immediately from mosquito bites. This year is one of the worst mosquito years in history in Colorado. On a recent hike in Rocky Mountain National Park, one could see and swat thick clouds of the beasts. I could never find references for absorption of ppt of sulfur in babies , or Eurax.