You should do as the prescription suggests. Also, keep a diary of when you change the dosage and how your daughter is reacting to the change. On the average, it takes about 4 weeks for our system to adjust to any medication dosage. Which is why i am saying keep a diary to note any changes of how she is feeling. There are side effects to consider too. Here is a link to know what she will go through, https://www.drugs.com/mtm/lamotrigine.html

Ask for lab work to confirm medication level in blood.
Please trust me when I say i have been the kid in this situation. Different medication.

I would call her neurologist first before you add or take her off of anything dosage she is on now.I know it can be frustrating!!!!sometimes it takes some adding or decreasing med before the Dr find the right balance.don't adjust the med yourself though.until you talk to her Dr I wouldn't do anything.