How do you say "They send their best regards to your whole family"
I am talking about my parents so it could also be "My parents send their best regards to your whole family"

Question

Answer

You can say:（うちの）両親がご家族の皆さんによろしくお伝えくださいと申しておりました。
[(uchino) ryoshin ga  gokazoku no minasan ni yoroshiku otsutaekudasai to moushite orimashita. ]

uchino= my 
ryoshin =parents
(go)kazoku = family  *go=> to be polite
minasan = all, everyone
yoroshiku otsutae kudasai = please say hello to (in polite way)
moushite orimashita = said (in polite way)

If you have other questions,  don't hesitate to ask me.