Prop. Treas. Reg. §1.125-6(b)(3)(i) requires a third party that is "independent of the employee and the employee's spouse and dependents" to provide information describing the dependent care service, its date, and the amount of the expense.
Under the DCA rules, care provided by an employee's child under age 19 or by a person for whom the employee or spouse can claim a deduction (a qualifying child or qualifying relative) does not qualify for reimbursement. But a DCA participant's child who is age 19 or over (and who is not a qualifying child or qualifying relative) may perhaps be sufficiently independent to qualify under the substantiation requirement. See IRS Publication 969 for more information.