"They have the most children."
Translation:De har flest børn.
It isn't wrong grammatically, but it sound too much like "They have most children" (like most children in the world or something). I guess it's weird that presence of "the" in English turns into absence of "de" in Danish and vice versa, but that's what it seems like for this example.
'Fleste' is used for "countable" items - you can count children. 'Meste' for uncountable - a Google search found these examples: liquids (milk, water); abstract ideas (advice, chaos, motivation); powder and grain (rice, wheat, sand); mass nouns (furniture, hair, transportation); natural phenomena (sunshine, snow, rain, weather); states of being (sleep, stress, childhood); etc.