Licorice and lemon balm both have some early research showing actions against hand, foot and mouth disease (Coxsackievirus A16 or Enterovirus 71).
Personally I’d also do the Immunity Support and the Respiratory-Shoofloo formulas. Both are here:
Immune System Formulas
Don't use herbs or combine herbs with medications or use them during lactation or pregnancy without talking with your healthcare provider.