Meftal Spas contains Mefenamic acid and Dicyclomine hydrochloride .
Since your son has consumed 20ml of it and having no symptoms, its good as of now.
Toxicity depends on the age and weight of the child.
If child is less than 7 years, you should take the child to nearest hospital.
If child is more than 7 years of weight more than 25kgs
You can wait and watch.
Next Steps
Look for toxicity symptoms like dizziness, vomiting, nausea, loss of consciousness, seizures
Health Tips
Better take child physically to a Peadiatrician