If your Doc's said it's okay, then I guess it's okay.
Or you can try Google searching it.
For me and my family per allergires, we use Claritin... or the generic brands of Claritin (MUCH cheaper). And compared to other brands, this is the brand that suits us the best. BUT, every person is different. Some brands did not work for me, but did for my Hubby.
For your child though, use ONLY the "kids" Claritin.
Claritin does NOT cause "drowsiness." Which is important.
I don't know about Zyrtec.
YOu may just have to try one brand first, then wait and see if it works. Give it a couple days. And if not, try the other brand.
You can also read about the ingredients/affects online... and if it is okay for pregnancy/children....if you want to just re-confirm on your own what your Doc told you.