A.S.
While I don't have personal experience with this, I'm a big fan of reading the information sheets regarding any medications I am hesitant on taking or giving to my daughter or husband. Here's the link to the info sheet for Zyrtec: http://www.pfizer.com/pfizer/download/uspi_zyrtec.pdf
To sum it up in layman's terms, studies have shown that Zyrtec relieves allergy symptoms (sneezing, runny nose, etc) in patients aged 6 months and older. As far as the side effects go, according to the clinical trial, the patients (6 - 11 months old) who took Zyrtec instead of the placebo were more irratable/fussy than the placebo patients. There was virtually no difference between the rate of drowsiness in each, though. In patients 12 months to 5 years old, however, the most common side effects were insomnia (9% Zyrtec vs. 5% placebo), fatigue (3.6% Z vs. 1.3% P), and malaise (3.6% vs. 1.8%). {Malaise is simply a feeling of discomfort/uneasiness}
Hope this helps you decide! And remember, there are side effects to every treatment, including homeopathic treatments.