A couple of questions to ask yourself:
Do you have pets?
Are your children in school or daycare?
Do they know how to use a public toilet?
Do they know how to wash their hands thoroughly?
Other children are carriers, and it is so easy for a child to become infested. An infested child who does not wash his or her hands and handles a toy can unwitttingly pass on worm eggs. Pinworm eggs under fingernails are viable for quite some time; it's a matter of transport from finger to mouth. A few suggestions about how to avoid recontamination