It's so hard to watch your child get so upset, even when you know you are doing the best thing for her in the long run. I feel you are right in treating early. It's not just cosmetic; you might be helping to stop it from getting worse.
I don't know where you are or who your doctor is, but I do know that not all laser treatments are equal. Applying the right settings and length of pulse is really an art, and some specialists are more skilled than others.
For such an extensive stain as you describe, it might be worth being evaluated by a team of specialists. The VBF has an annual conference and clinic every year where teams of specialists will examine her for free (well, included in the registration fee). This year will be in Irvine, CA on Oct. 10, 2015. Next year will be in NYC. Let us know if you decide to go.