You will have no idea where they came from. They could come from anywhere. The eggs can live anywhere. You did the right thing in treating the whole family. Mebendazole (ovex) should be used to treat the whole family, then again in 2 weeks` time. It`s a good idea to wash all bedding, towels, underwear and PJs in a hot wash to kill off any eggs otherwise re-infection can occur. Apparently scrubbing under the fingernails with a nail brush is a good idea (I always wondered why my mother told me never to bite my nails) as the eggs can live there. If you can treat it and manage it I personally don`t think there is any point in going to the GP