"The six boys are going to eat rice."
Translation:Die sechs Jungen werden Reis essen.
I've only seen numbers get an "-en" ending added in the dative plural (z.B. "mit den dreien"), but even then it is pretty rare that I have seen that ending added.
Numbers are almost always left uninflected.
The 'helping verb' that German uses to form sentences in the future tense is werden - see the lesson tips page if you need more information about the future tense.
The conjugation here needs to be 3rd-person plural, the same as for "they", because we are talking about multiple boys. That conjugation is the same as the infinitive, i.e. also werden.
Die (sechs) Jungen werden Reis essen
Sie werden Reis essen
Der Junge wird Reis essen
Er wird Reis essen
Du wirst Reist essen
Ich werde Reis essen