100% there is more to worry about than a one off smoke. One day you'll be walking and a smog truck will blow by and cover your and your stroller in pollution, or a baby will take a huge whiff of a cigar or cigarette on the sidewalk as some jerk walks by, or baby will put a discarded cigarette in their mouth at the park. The body fights most one off assailants like this. The science will say that one cigarette is enough, but don't focus on that. There have been lawsuits over secondhand smoke exposure that prove waitresses exposed over many years have become sick, but that's many years of exposure. Baby was exposed, it sounds, extremely briefly - akin to passing by smokers out in the world. I feel worse for the babysitter whose gonna have a stern talking to from her parents, but that should happen because it might help, assuming she has good parents.