How much fluoride will my nephews absorb through there skin if there 1ppm fluoride means 120 milligrams in 30 gallons if there 1ppm fluoride this means 120 milligrams in 30 gallons how much and how long to ingest that much rat posion in the city water cal

Very very little: Like a lot of things (vitamin c, iodine, iron, oxygen) Fluoride is safe and helpful when used in the proper small amounts, and like these other substances can be toxic in very high amounts. You can not realisically, and rationally compare the two scenarios, the effects are totally different. They will absorb no measurable amount of Fluoride though their skin from city water containing fluoride.