Magic bands don't have anything loaded on them. They are a band with an RFID chip in it. This chip corresponds to a number that accesses your My Disney Experience information. So, that means that it simply accesses the ticket and reservation system information that is all linked to My Disney Experience.
So, you won't be able to load any kind of money on them. If you want her billing separate from yours, you'll have to buy her Disney Gift Cards as previously suggested or she'll have to carry cash with her.
If it were me, I would hold on to her cash and have her charge her food to the room using her band (which is essentially what you all do when you use your bands to charge). Then, each evening, I would have her reconcile what she has spent and give you the cash. This will allow for three things - she will keep track of what she spends daily so she doesn't go over what she has (GREAT lesson); you can make sure you are all on the same page so all costs are covered; and at the end of the trip, you can use the cash she gives you to pay her part of the bill when you settle it at departure.
One possible drawback to this is that she may spend or purchase more than she has and that is a risk you take when using this particular method. If you want to avoid it, I think gift cards are the best way to do so.
