I do not know what arrangements were made, but if you are not paying any interest, I would stick to paying your parents instead of getting a loan from a lender. Lenders charge interest.
If you are to pay your parents a balloon payment at some point in time and you will need to get a loan, then all the payments should go right into the equity of the home as you have stated. Your parents would need to write up a document reflecting this and you should have little problem obtaining the loan as long as the $12K is at least 20% of the homes value. Also, the payment history, as long as it was consistent, should also show a good payment history.
Hope that helps,
Terrence Charest, e-Pro