Is it okay for a preschooler to wet his pants once in a while?

Like what Idza mentioned, if it is just a one-off incident, there is no real need to worry about it. Accidents do happen. If it is a recurring issue, other than emotional causes, it could also be due to physical factors. A urinary tract infection, diabetes, or gastrointestinal issues, like constipation may cause a child to wet his/her pants. Urinary tract infections and constipation may put pressure on the bladder and the child may not be able to handle yet. As for diabetes, frequent urination is one of the first signs (together with extreme thirst). Therefore, the child may be drinking much more and needs to go to the bathroom often, and sometimes, he/she may not be able to make it in time. If you are worried, bring your child to a doctor to find out if it is a physical issue that is causing it. For more information, here is a useful article: http://www.newkidscenter.com/4-Year-Old-Wetting-Pants.html