I would continue to spell out "six" and "seven;" the usual rule in formal writing is to spell out small numbers. I'd use hyphens, thus: "six- and seven-year-old children" Spell the word "recognise" if you are writing for a British English audience, "recognize" if U.S. English. Thus "How six- and seven-year-old children recognise emotions" or "How do six- and seven-year-old children recognise emotions?" are both correct. The simple statement is plainer and more formal. It also is more ambitious: it promises an answer to the question. The question form is informal. For a magazine article it would be better because it involves the reader. For an academic paper, I would look at the table of contents of a few issues of the relevant journal and see what tone other writers are using. Of course, it also depends on the content of the paper, and how well it actually answers the question.