"The child sees the orange juice."
Translation:Das Kind sieht den Orangensaft.
In German, articles (der, den, einen etc) depend on the gender and the case of a noun. "Saft" is a masculine noun and here it is in the Akkusativ case. A word is often in the Akkusative case when it is the object of the sentence though there are a few verbs and prepositions that can change that. For masculine nouns in the Akkusativ case you use the article "den". In the Nominativ case, when the masculine noun is the subject, you use "der". Same with "ein" and "einen". In Nominativ you use "ein" - "ein Orangensaft" - in Akkusativ you use "einen" - "einen Orangensaft". Here is an overview of the definite articles: https://en.wiktionary.org/wiki/den#German and here of the indefinite articles: https://en.wiktionary.org/wiki/ein#German