Nothing to be done,as a pinch of detergent will not be harmful,but are going sure only a pinch was ingested?but better take care to keep these substances out of reach of child, children have exploratory Behaviour and put everything in mouth...
Next Steps
give water or milk to child it will cause dilution of the detergent if it has been ingested
Health Tips
watch for any respiratory distress if detergent was inhaled,if 4-6 hours passes and nothing new happens, no need to worry,if any symptoms, visit the doctor