I'm not expert on the topic but it's a highly contagious and just takes a small droplet from someone who is infected. Maybe your daughter is taking extra precautions so that your granddaughter doesn't catch it?

You can get hand, foot, and mouth disease by:
-- Contact with respiratory droplets containing virus particles after a sick person coughs or sneezes
-- Touching an infected person or making other close contact, like kissing, hugging, or sharing cups or eating utensils
-- Touching an infected person’s feces, such as changing diapers, then touching your eyes, nose, or mouth
-- Touching objects and surfaces that have the virus on them, like doorknobs or toys, then touching your eyes, nose, or mouth