Translation:These two people are married and they have two babies.
So if it's just the plain number + partitive it's conjugated into singular, but if it's modified with a plural determiner, it's plural?
Kaksi ihmistä on naimisissa. vs Nämä kaksi ihmistä ovat naimisissa.
Is it correct? And how do you know whether you add the singular or the plural determiner?
Yeah if the number is preceded by "nuo" or "nämä" stuff after it is conjugated as plural, because it makes the phrase definite.
In some other cases when the phrase is definite it can work the same way, for example:
"Kolmetoista maata pääsivät eilen sopimukseen."
"The 13 countries reached an agreement yesterday."
For whatever it's worth, this would be a rather awkward thing to say in English unless you're emphasizing that the offspring are truly babies and not, say, four-year-olds. In a neutral context I'd be far more likely to say "children" rather than "babies" (even if they really are babies).
Is the Finnish here really being specific about the kids' approximate ages (i.e. they ARE babies), or would this be a more neutral statement?