Hand, foot and mouth disease at preschool. Should my kids just go catch it and be immune?

Probably will. Contagious virus which is harmless. Treatment is supportive. Tylenol (acetaminophen) for fever, topical antihistamines/steroids (otc) for rash, and antacids for mouth rasr. Return to school when afebrile.
Not a bad idea. A mild case may not hurt.