What's the best pain medicine for a child with toothache?

Some ideas; dentist. Acetaminophen (such as tylenol) or Ibuprofen (such as advil (ibuprofen) or motrin) may help temporarily. You can apply a topical anesthetic such as oragel to the gum. However, it is important to see a dentist as soon as possible as the pain could represent an injured or diseased tooth, or abscess (infection).
DDS. Tylenol or Motrin will help, but not solve problem. See DDS.
Best med is dentist. Childrens age appropriate dose of tylenol (acetaminophen) or Motrin may help with symptoms but if your child has a tooth ache it either needs to come out or needs a big filling and I recommend you see your dentist to take care of the prob. Pain meds mask the problem temporarily, they do not fix it.
Your telephone. Best thing you can do is call a Dentist or preferably a pediatric dental specialist, a Pedodontist, for immediate care. OTCs will not cure your child's infected teeth. Don't delay, call now.