=enterobius. Common in young children, probably due to hygiene issues. Also called pinworms. Small (1-2cm), like white threads. Can be intensely itchy, usually in night when worms emerge to lay eggs.
Mebendazole effective at killing live worms in gut, not effective if eggs in vagina. Licensed from age 2 but can be used off license below that. Does not kill eggs however, and main issue is reinfection so consider treating whole family, with repeat doses after 2 weeks when any eggs will have hatched, and follow NHS Inform advice on hygiene.
Sometimes you just have to keep repeating treatment over months/years! Mebendazole is harmless, although symptoms may be caused by clearance of large worm burdens.