"Ein deutsches Kind isst Obst."

[Ehsan]

lost a heart for using "fruits" instead of fruit. why can't I use fruits here?

[brasil2004]

Obst ist ein Singularetantum, dh. es gibt kein Plural von Obst im deutschen! the same=Milch (milk)

[asac3000]

thank you brazil2004 for the answer in German. Love this way of answering when it remains simple enough to be understood.
I felt proud when noticed I understood your explanation in German. Great!

[xiongnu1987]

Fruit is uncountable in English too.

[mirage014]

Why is it "deutsches" in this sentence?

[vinaysaini]

thanks brasil2004.
to mirage014: "deutsches" here functions as an adjective mentioning the nationality of the child (a neuter gender in German).