From medicines.org.uk:
Cetirizine Hydrochloride 10mg Tablets
Toxicity: Limited experience of overdosing. 20 mg to a 2 year old, 30 mg to a 3 year old and 40 mg to an 11 year old did not give any symptoms. 60 mg to a 4 year old gave mild intoxication, 400 mg to a 14 year old gave mild symptoms while 400–500 mg to an adult gave no symptoms at all.
Symptoms of overdosage reported with antihistamine substances: Somnolence, unconsciousness and/or excitation (principally in children). Ataxia, tremor, headache, hallucinations, seizures, dry mouth, flush, hyperthermia, mydriasis, urine retention tachycardia and in the case of massive doses, possible fall in blood pressure and arrhythmias. Nausea and vomiting. Also extrapyramidal symptoms are possible. Cetirizine hydrochloride has a low sedative and anticholinergic effect. Sedation can be a symptom of overdose, it can occur after a single dose of less than 50 mg.
Treatment: At the present time there is no specific antidote. Experience of overdosing is limited and no severe intoxication has been reported to date. Primary treatment should be gastric lavage, if justified and charcoal. Symptomatic treatment should be instituted in the case of acute intoxication, such as diazepam for seizures or acute dystonias.