According to the IRS, you must actually pay the mortgage to be able to claim it.
Who gets to take the deduction?
You do, if you are the primary borrower, you are legally obligated to pay the debt and you actually make the payments. If you are married and both you and your spouse sign for the loan, then both of you are primary borrowers. If you pay your child's mortgage to help them out, however, you cannot deduct the interest unless you co-signed the loan. You can however make gifts to them so that they can make the payments and deduct the interest.
IRS Publication 936: Home Mortgage Interest Deduction.
