Permethrin is medically used for head lice and scabies for very short durations. A very small amount is used (1 to 5%) and hence the amount absorbed is very small. Transitory skin contact by breast-feeding mothers does not cause any problem.
Chemical fertilisers (such as urea) are sold in concentrated forms. Hence children should not touch them because of the risk of ingestion. However once the fertilisers are sprayed and watered, they get diluted and hence less harmful. Children should be taught to wash hands before eating.