Vertaling:Het kind en het meisje
It depends on word gender. Masculine and feminine words get de, neuter words get het.
So your obvious next question would be: Okay, how do I figure out the gender of the word. And the answer to that question would be: "hardly". There are some extensive rules - which undoubtedly aren't foolproof either. So I think you'll just have to pick it up as you go along.
Roughly the only rule I know on the matter, is that diminutive forms of nouns [shown by the addition of the suffix -je] are always neuter. So the boy is de jongen. However the little boy is het jongetje. Occasionally [for instance in this case], the suffix is extended from -je to alternate versions such as -tje or -pje or a small number of other, rarer, possibilities. That's done for pronunciation. Anyway, whenever you see such constructions, the word will be neuter - and thus you'd have to use het.