If you add this paragraph to Alpha's suggested letter I would change that paragraph to something like this:

Please remember that this is a small in-home daycare where the children play together closely. If your child is sick please keep them at home and take one or two days off work to care for them so that all of the children in the daycare are not infected. It is my job to protect all of the children and you would not want me to allow a sick child into care who could infect your child.