the name the entire family uses now. I don't know if they had to do anything legally to make the change

Yeah, as I said above, I don't believe it's a problem calling kids whatever you want (including surname) - you just register their names accordingly. As regards the parents, there would normally be some legalities involved (you change your name by "Deed Poll" in the UK), probably slightly more straightforward for the woman than the man due to the relevant authorities/companies having had more practice on that front (total supposition, this, but bet I'm right ).

Once the couple have changed their names, they then need to notify everybody with whom they ever have dealings, especially banks. So give it another year or so for the name changes to percolate through for both the man and the woman. Then even junk mail will start coming through with the right names. Maybe.

I have a friend who wanted to change his surname and adopted a far more cheap and straightforward route - he simply declared to all and sundry his wish to be known as the new name. This doesn't seem to have presented him with any problems, even with banks etc. And he doesn't get post addressed to his old name with "k/a newname" next to it, either, which surprised me a bit.

Fisk