What can I use for immediate relief for a child toothache?

Infection. Toothache probably means infected tooth. Use Children's Motrin (ibuprofen) EXACTLY as directed on package, and use cold compresses. Pain killer and compresses may reduce symptoms, but will not cure the infection. Have to see Dentist IMMEDIATELY for root canal treatment, or extraction and space maintainer. Your Dentist may want your child to see a Pediatric Dental Specialist.
OTC Med. An OTC med like Tylenol (acetaminophen) may help. Ice over the affected area may also provide some relief for some toothaches.
Ibuprofen helps. I give them Ibuprofen and have their dentist evaluate them as soon as possible.