J.W.
I teach preschool and spend much of my work day telling little people to get their fingers out of their noses, it's germy. I also let them know I won't let them touch me until they clean their hands because I don't want to get sick. I have a hand sanitizer available at all times.
I found a great little song I sing to them when I see a finger exploring the nether regions of their noses..
"Don't put your finger up your nose,
Your nose, knows it's not the place it goes.
You can sniffle, you can sneeze
But I'm asking you please..
Don't put your finger up your nose!"
I agree, it is gross and disgusting.. but use your sense of humor to get your point across! I would stress the germ part and that you can't let them touch your kids or you if their fingers have been in places they don't belong.