Can zyrtec or claritin (loratadine) cause a child to have diarrhea?

It's very unlikely: It's very unlikely for these antihistamines to cause diarrhea, so look for other possible explanations. If the diarrhea is transient, there's a good probability that it could be viral.

Not likely: Almost any medicine can cause almost any side effect, but as dr. Sanico said, it would be extremely unlikely for zyrtec or Claritin (loratadine) to cause diarrhea.