It is totally about the money. I think she figured since you were going to the school anyways that it was no bother to just take her two as well and so why pay you were already doing it so it isn't like it is an extra service at least in her eyes. What she needs to realize is that you are being paid for the service by the other children and if she wants the service she needs to pay for it too. Maybe if you even explained it to her this way she would admit it is the cost or at least realize that it is an add on item.