Since they only weigh 7 pounds I would try to get them to throw it up. Allergy Meds are pretty universe from human medications to cats and dogs. Half a 10mg tablet then I would say don't worry to much, but the amount he took I would try to make him throw it up. When I worked at a vet we would give boarding cats and dogs Meds like this all the time but based on weight. He might get hyper but I would keep a close eye on them if you can't get them to throw it up. Take them to the pet hospital or call a vet and ask if he seems like they are acting different. I still would call a on call vet, or ask to talk to one over the phone, not just someone at the front desk.
Sorry misread her for a boy, just insert she and her
Personally, I would be rushing my cat to the vet ER for treatment. Might be overkill but better to be safe than sorry IMO. Zyrtec is commonly given to cats for allergies but the dose is much smaller, usually no more than half a pill. I'm not sure what effects might happen if a full pill was ingested by a cat. At the very least, you can call animal poison control for advice (the ASPCA's poison control number is (888) 426-4435 ) or wait until your regular vet is open and call for advice (if they're open on weekends. If not, call around to other vet hospitals).