When you see (sic) it means that something has been copied exactly even though it is a mistake. She should have said children do because 'children' is a plural subject.
Here into refers to something you are very interested in or like a lot.
Example: I'm really into learning English.
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Notice that the short form of because is spelled with an apostrophe - 'cause. Follow the link to find out more.
just about; almost
Example: English is pretty much the hardest language to learn.
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