If you file a tax return for any reason, you must report all your taxable income, regardless of the $400 limit on self-employment.
However, I think consideration here should also be given to the question of whether this is "other" income like a hobby, or self-employment income from "an ongoing trade or business."
https://www.irs.gov/businesses/small-businesses-self-employed/business-activities
See here for a 9 factor test:
https://www.irs.gov/faqs/small-business-self-employed-other-business/income-expenses/income-expenses
