If you wish to deduct childcare costs, you must not pay the money in cash, but must transfer it. The tax office may require the invoice and, if applicable, the transfer receipt as proof. A receipt from the recipient is not sufficient.
Note: If the employer provides tax-free benefits for pre-school childcare, the deduction for special expenses must be reduced by the amount of these benefits, as the deduction for special expenses requires expenses that actually and definitively burden the taxpayer financially (BFH ruling of 01.09.2021, III R 54/20).