My daughter had to have stitches on her forehead when she was six. Will the scar go away eventually?

No. When the skin is disrupted, a scar is the only way that it will heal. There are degrees of scarring. It may be hat the scar will become barely perceptible and thus very cosmetically acceptatable but the scar will still be there. Some scars hypertrophy and are unsightly and can be treated in many ways to improve them. Still there will be a scar. Takes about a year for a scar to mature.
Improvement likely. A better ? Might be will the scar be very noticeable, as a scar is the result of healing and will always be present. Most improve with time and the initial redness of fresh scars tends to resolve in 6-12 mos. Many can be improved by use of creams or silicone in the first year; older scars may be revised w/ other techniques. Eval't' and mngt can b useful as can not making the child self-conscious.