Try, "I have been -ing", or contracted "I've been -ing":
EX: I've been working here for two years. (Span of time)
EX: I've been working here since 2002. (Specific date, day, year, ect.)
The phrase It is been is incorrect. The correct form is "It has been":
EX: It's been (i.e., It has been) two years since I started with this company.
Speakers often contract "It" and "has", giving "It's", which looks like, "It is".
Your other examples
It was the children who broke the window. (OK. Note, 'was')
=> The children are the ones who broke the window. It is they who broke the window ~ It is them who broke the window.
These are the children who broke the window. (OK)
They are the children who broke the window. (OK)
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