Giving anti-histamines to children

Question:

Hi, my friends and I were discussing the antihistamine drug zyrtec and the following questions popped up:

- one of our group plans to systematically gives her 2 year old zyrtec throughout the summer to control his reaction to mosquito bites. Is this safe?

- another mom says she will use zyrtec to help settle her daughter on the plane as it has drowsiness as side effect. Again, is this safe?

It seems crazy to me to use any kind of drug so liberally but perhaps I am wrong?

Thanks!