For a 12 kg child, with ibuprofen 100 mg per 5 mL:

Give 6 mL by mouth per dose, preferably with food or milk.
Repeat every 6 to 8 hours if needed. Do not give more than 3 doses in 24 hours unless a clinician has specifically advised otherwise.

Do not give ibuprofen if the child is under 3 months, is dehydrated or vomiting significantly, has kidney disease, a stomach ulcer/bleeding history, NSAID allergy, or chickenpox. Use caution with asthma unless ibuprofen has been tolerated before.

Tooth pain needs a dentist review soon, since painkillers do not treat a dental infection. Seek urgent dental/medical care today if there is facial swelling, fever, pus, difficulty swallowing/breathing, inability to open the mouth, or the child seems very unwell. A MedlinePlus child dosing table lists 5 mL for the standard 10.5-15.5 kg band, so if this is an over-the-counter product, follow the specific label or pharmacist's advice if it differs.