Actually Phenergan (promethazine)
tablets are either 10 mg or 25 mg.
It's true that the liquid form is 5 mg / 5ml .
But I agree most strongly that medication of this sort ( it is, after all, in the same chemical family as some of the major tranquillizers - known as the phenothiazines ) should NOT be given to pre-pubertal children without at least some medical advice.