I struggle with this as well, but having been in the nanny business before, I am in the habit of washing hands myself. I do forget to insist oftentimes, and they frequently protest (except getting dressed and ready for gym-- I just wish they did it without a fight after).
My method when they refuse and I think they should? I take a page from dear old Grandpa and scare the daylights out of them by telling them that is almost always how they get sick. We don't get sick very often (we homeschool--that helps a tiny bit, having less contact with dozens of kids everyday) and they hate being sick with a passion.
Now, if only I can remember to remind them, especially before eating.......
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