Congratulations to your child, and to you on your successful child rearing.
What are you trying to achieve here, other than not having 8% interest go out of the family? If all you are trying to achieve is not having 8% interest go out of the family then you pay off the loan and enter into a contract with your child that will mean that they pay the same amount to you as to the student loan company, on the same terms.
But your other questions are about other things: whether you make a gift to your child and if so how much and on what terms and conditions. And that raises all sorts of questions that we have no idea how to answer, as they depend on individual personalities and behaviours, on family dynamics, and on a full appraisal of the financial situations of everyone concerned, including any other members of the family with a potential interest, such as any siblings to your lawyer child.
I'd be tempted to say: if you have an educated, responsible child and money to spare why wouldn't you make a gift of the repayment?
Other mustachians will be of the bootstraps/pull yourself up persuasion.