Addressing a sympathy card

I would send a card to each of them

Yes, address it to the wife / widow as pps have said.

I would do Mrs XSmith and mr Y smith or if you know them very well send a card to each

Can Mr Google help? Debretts might be helpful too?
I believe that a widow should be Mrs J Smith (or whatever her forename initial is) and her son Mr J Smith as per the son's forename initial & surname.
I would write:
Mrs J Smith, Mr J Smith
If you use "and family" she may think to herself that they are not the family they used to be anymore & might be upset at that thought?
It's a difficult one but I'm sure Google can help. It's kind that you are taking time to get it right. So many folks wouldn't even send a card these days. One would likely get a text message!

Thank you, DH made me overthink it, but you've all reassured me.

Mrs. J.Smith and family.

I agree with Sardinia and lemongrove.

I sent two separate cards and letters when my friend's husband died. She told me afterwards that I was the only person who had sent anything to her daughter alone. She really appreciated it.

I agree with Sardinia

Think I would just put Mrs J Smith. She will share it with her son and you’ve no doubt written the message inside to both mother and son.

Except the Master could be in his 50s!

I'd go with Family.

Mrs Jane and Master James Smith.

Mrs Jane & mr James Smith

Don’t put anything else on the envelope

What about "The Smith Family"? Simple and accurate still.