D.G.
I had the same problem with my 18 month old recently. The only way I could get any antibiotic down her was by holding her down and squirting a little at a time in the back of her mouth via syringe.
I called the nurse at the pediatrician to ask if there was a better (and less traumatic) way of administering the medicine. She said I could try mixing the medicine with jelly or chocolate syrup.