"Meneer en mevrouw De Jong hebben geen kinderen."
Translation:Mr. and Mrs. De Jong do not have children.
I suppose they marked 'Willem/William' correct because it's a first name and they're written different in different languages but 'De Jong' is a last name and you can't translate last names because that's too confusing, I hope this explanation helped you. If you have any more questions, don't hestitate to ask me
Not exactly. The bit before the name is known as a tussenvoegsel and it isn't capitalised in the Netherlands unless it is directly preceded by a title.
Vincent van Gogh Meneer Van Gogh
For added fun when names are alphabetised the tussenvoegsel is ignored in the Netherlands, Mevrouw De Jong is filed under J, not D.