My kittens think my stairs is their bathroom!?

I just got two kittens from my mother in laws. they were outside kittens.they have been pretty good about using the litter box, but there are a few times i have had to clean some up off of the stairs. (it's not the hard stuff either). I understand this could be from me recently worming them *(so i spray with lysol after i clean to disinfect). every time i let them out, there seems to be an accident. how can i help them so they can come out and be free more often ?