Please advise what is the best painkiller for a child with toothache?

Pain. For teething childrens Motrin and follow the directions exactly! for decay the best thing is to see the dentist and get the cause of the pain treated before any damage occurs to other teeth or unerupted teeth.
Ibuprofen. Ibuprofen is the most effective non-narcotic pain killer for children over 1 if used EXACTLY as per package instructions. But pain meds will not cure the reason for most toothaches, infection. If your child has passed his/her1st birthday, he/she should be seeing a Dentist regularly. Please call the Dentist for an immediate appointment.
See a pedodontist. Take OTC painkillers may provide some relief until you can get to your pedodontist. Tooth pain is caused when the nerve root of a tooth is irritated by infection, decay or trauma. The good news is the prognosis is good with appropriate dental care.