Wait until they receive a penalty letter and then try this:
ABC qualifies for an exception to the penalty you charged under IRC Section 6698(a)(1), because all two (2) partners, XZ, 222-22-2222, and YZ, 333-33-3333, reported all their shares of income and deductions timely on their joint 2019 income tax return Form 1040. Therefore, the partnership meets the criteria specified in Rev. Proc. 84-35, which says in part: “A domestic partnership composed of 10 or fewer partners and coming within the exception outlined in section 6231(a)(1)(B) of the Code will be considered to have met the reasonable cause test and will not be subject to the penalty imposed by section 6698 for the failure to file a complete or timely partnership return, provided that the partnership, or any of the partners, establishes, if so required by the Internal Revenue Service, that all partners have fully reported their shares of the income, deductions, and credits of the partnership on their timely filed income tax returns.”
We trust that you will remove the penalty due to reasonable cause for small companies.