Call poison control. Usually too much zyrtec (cetirizine) would not cause any harm. If you gave two to three times the recommended dose i wouldn't worry about it. Is the child on any other medications which might interact with the zyrtec (cetirizine)? There is an 800 number in your area which you can call 24/7 for questions regarding overdose or accidental ingestion of cleaning products etc.
It depends... It really depends on how much is given. If a small overdose is given typically no changes occur. If a large overdose, say 3-4 times the normal dose, then one might see some side effects that we consider anti-cholinergic in nature: flushing, dryness, inability to urinate, etc. If you have such concerns, i would recommend calling a poison control center or visiting a local er.