You have to report rental income if you actually receive it. If you do not charge rent to your children, you do not have to report rental income but you cannot deduct any rental expenses.
If you rent your property below fair market value, the IRS considers that you do not rent your property to make a profit. In this case, you can deduct your rental expenses only up to the amount of your rental income. You cannot deduct a loss or carry forward to the next year any rental expenses that are more than your rental income for the year.
Mortgage interest and property taxes paid are claimed in Schedule A as for your residence.
Please refer to this IRS document, especially the section on Not Renting for Profit on page 16 on how to report income and expenses:
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