If you are going to borrow it, why not put the re-paid money into a Junior ISA in her name - that way no one can access it until she is 18, and if it grows sufficiently, she will have a nice nest egg to start her adult life.
You may also want to put a note with any wills you have, stating that you have borrowed it and she is to be repaid out of the estate.
My DC have had quite a large inheritence recently, (out of the blue, from a cousin of mine) DS is 18 so has got his, DD is nearly 16 and it is still with solicitors. We are due to meet them next week, at their suggestion, with a view to myself and DH taking over the managment of DD's money for the next 2 years, as solicitors fees would otherwise reduce the capital.
Slightly different I know, due to DD's age, but solicitor has made clear he won't do anything unless she gives her full agreement and then we will all have to sign paperwork permitting us to look after the money for her. At an initial meeting I had with him a couple of months ago, I was also quizzed on whether we all lived together, whether DH and I were a couple etc.
Fortunately she already has an ISA she can top-up, and we are investigating other accounts that a 16yo can have, to get as much as possible into her name.
However, if some of the money does stay in our name, and the worst was to happen, at least there will also be a legal paper trail to show it is her money.