happened with our children, we used Ovex I think. Two doses spaced a fortnight apart if memory serves.
The first dose deals with the ones which are there already, and they will come out in the stools. The second dose is timed so that the eggs left behind have enough time to hatch and grow but not mature and start laying more eggs before being dealt with.
Oh, and although you don't have them, you are going to have to take the treatment as well as a precauition.
In the meantime, do your best to keep your lass from scraching her behind - that, especially if she does not wash her hands thoroughly, has the potential to transfer more eggs from there to her mouth, starting the cycle all over again.
The NHS Choices page about them is
http://www.nhs.uk/conditions/threadworms/Pages/Introduction.aspx