Hi Anhnha,
I think the sentence is parsed as follows:
There are twelve including the children.
There-expletive (pronoun)
are-intransitive verb
twelve-subject (noun)
including the children-adverb modifier (prepositional phrase)
including-preposition
children-object of preposition
the-adjective modifier
I don't think the other two examples are valid sentences. Can anyone else confirm this.

Student or Learner
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