You are correct, this does belong on your tax return. However, you do not need to report this as self-employed earnings. A gift from an employer belongs on a W-2 as wages. Since it sounds like you received cash with no reporting, you can add this as other income on your tax return.
Navigate to: Federal > Wages & Income > Less Common Income > Miscellaneous Income > Other income not already on a W-2. Select Earned income not reported on a W-2 or 1099 and continue. Now, follow the interview to report the gifts.
See more information from the IRS: De minimis fringe benefits.
