Is it unsafe to expose a baby to third hand marijuana smoke? (ie. The smoke absorbed on a persons clothing/skin who just smoked it)

I think that's a YES. Honestly speaking, if it's ONLY once then, risk of harm probably no worse than exposing the baby to breathing normal FRESH air of the suburbs. However, in my experience, babies who are already exposed to adults within their circles whose clothes are impregnated with odors and smoke from marijuana, as well as tobacco, etc. are extremely likely to be exposed multiple times in their lifetimes.
Third hand smoke. The amounts of chemical in third hand exposure would be incredibly small and in my estimation completely incapable of being measured after contact. There is no significant risk here. Would certainly be more concerned about a first hand accident caused the stoned individual than the passive transfer of trace adsorbed chemical.