At times we don‘t use an article in German. That is then called zero article or Nullartikel (m).

Ich habe xxx Kinder. Die Kinder sind groß.
I have xxx children. The children are big.

Unfortunately, there are differnces between English and German when it comes to the Nullartikel. Here's a little insight into those differences. You don't have to memorize this at all. You'll pick this up as you go without any addional effort. This is merely FYI (for your information).

Different usage I

Article in German but not in English:

Ich fahre mit dem Bus. >> TRANSPORTATION
I go by xxx bus.

Du wohnst also in der Lindenstraße. >> STREETS & SQUARES
So, you live in xxx Lindenstreet?

Ich bin noch auf der Arbeit. >> WITH ARBEIT, KIRCHE, SCHULE, BETT
I am still at xxx work.

Different usage II

Article in English but not in German:

Ich spiele xxx Klavier. >> INSTRUMENTS
I play the piano.

Ich höre xxx Radio.
I listen to the radio.

Ich bin xxx Mechaniker. >> PROFESSIONS
I am a mechanic.