I always tell mine just to bring themselves, thought they sometimes bring a bottle of wine anyway, especially if it is a 'special' meal.
In the early days of our sons visiting us as part of a couple, they would offer to help wash the dishes afterwards, but I always thanked them but said there was no need - my parents always turned down my offers of help when we visited with the children, and it was so nice to have a break from such chores that I now enjoy giving my own children that break.
One DiL is from another country, where it seems you wouldn't dream of turning up totally empty-handed at some else's home. It took her some time to accept that we genuinely don't expect her to bring anything here, but she got there in the end 
Her grandma, on the other hand, on her occasional visits, can't bring herself to break with that practice when here, and insists on bring something, even if it's just a banana or a single wrapped chocolate.
I know that both of my sons and their wives take along a good amount of food and drink when visiting friends.