A loan is made pursuant to a contract or agreement--the terms under which it is made. Those terms, once agreed to between the parties (the lender and borrower), may only be changed either if (1) both parties agree to change them; or (2) the initial agreement provided that the terms could change under certain circumstances. (For an example of this, consider an ARM--by it's very terms, it contemplates that the interest rate changes over time.)In the case you describe, the terms are whatever is set down in the written documentation. Only if that documentation provided for changes in interest or fees could they be imposed; otherwise, the parent may only charge what the parent and child originally agreed to, as reflected in the written documentation.