Menses occur due to periodic shedding of the lining of the uterus.
Normally this lining which is weak in the early part of the cycle, gets changed after ovulation and thus when it is shed, it comes off in few days, and the flow is definite, 5-7 days max
During puberty the girls have anovulatory menstrual cycles, where in the lining of the uterus remains "unchanged" due to the absence of ovulation......
As a result the lining of the uterus is "weak" and it can shed not as a whole but in bits and pieces.
so what your daughter is suffering is called "pre menstrual spotting" and it is expressed as light menses and when the real menses start, the flow increases.
Its quite common. But finally her days of bleeding are increased.
This transition from anovulatory to ovulatory cycles happens on its own, and there are NO medicines for it.
If she needs to use pad for longer number of days, then you can stop the pre menstrual spotting by taking medicines.
For this you have to visit a Gynecologist , get some hormone evaluation done e.g.
Thyroid function test and others, and a ultrasound scan.