My five year old daughter negotiated $20 in back allowance (we hadn't paid her allowance in a while, and she's a better negotiator than me) and then donated that money at her school to a charity for sick children. Assuming the charity is qualified, do I deduct the contribution from my own taxable income? Or is the allowance payment considered a gift or income to her, and so she would deduct it on her own tax return if she had one?
No, you may not deduct the charitable contributions of your children. The Nest covers this in detail:
The IRS only allows you to deduct charitable contributions that you personally funded, whether the contribution was made in your name or in someone else's. If your child or dependent makes a donation to a charity, you are not allowed to claim it as a tax deduction. This is true even if your dependent does not claim the contribution on his own tax return because he opts for the standard deduction rather than itemizing or claims exemption.
Now, had you constructed the transaction differently, it's possible you could've made the contribution in your child's name and thus claimed the deduction. Allowance is technically a gift, and if she agrees to forgo allowance in exchange for you making a contribution, well, the IRS can't really complain (though they might try if it were a large amount!). Contributions in the name of someone else, but funded by yourself, are deductible:
[Y]ou can deduct contributions you make in someone else’s name. So if you donated a certain amount of money to XYZ charity in your child’s name, for example, you would be able to deduct this amount on your taxes, as long as the deduction requirements are met. You will need to keep accurate records of the payment along with the receipt from the organization to prove you financed the donation.
With the new tax code of 2018 we have
For contributions of cash, check or other monetary gift (regardless of amount), you must maintain a record of the contribution:
A bank record or a written communication from the qualified organization containing the name of the organization, the amount, and the date of the contribution.
i.e. all cash amounts must be documented from the charity. I'd ask you in return, did the school issue proper receipts?