What kind of allergy medicine is safe for a five year old? Is zyrtec (cetirizine) ok?

See MD. Before any medications are taken one should always discuss with their physician first.
Yes. Zyrtec (cetirizine) is ok. The usual dose for a 5 year old is 5ml (5 milligrams) of the liquid zyrtec (cetirizine) or one chewable 5 mg tablet.