Good advice from imalurnin - pinworm eggs can live in the air for two days without a host. They are the "head lice" of the parasite world - because they are so easily able to reinfect their host.
The females come out at night and lay their eggs on the anus, underwear and bedclothing. The host scratches and the eggs are on their hands. In children, reinfection is very common because they scratch then later on in the night they might put their fingers in their mouths.
During a parasite cleanse for removal of pinworms, the bedclothes and underwear must be washed in HOT water everyday. This will help get rid of the bulk of the eggs. Giving the child a wash down every morning will also help get rid of the microscopic eggs that might be on the anus and legs (hands too).
Also, disinfect the bathroom surfaces to get rid of the eggs. Pinworms are so contageous that usually the whole family can get them quite easily from the infected person. Your child might also be going to school or daycare with an infected child and that is how she keeps getting them.
This is why I always recommend bi-annual parasite cleansing. Parasites are very easy to obtain and cleansing twice per should be a part of life for everyone over the age of 2. Wormwood is VERY DANGEROUS for children under the age of 12 and under 100 pounds. It is a narcotic and can cause mental disturbances in the wrong doses - there are other herbs available that will work just as good for parasite removal in children - just without the narcotic side effect.