Surgery vs Observe. Most nevus sebaceous do not develop into skin cancers. However, it is estimated that ~10-15% develop a skin cancer called basal cell carcinoma (bcc). Because of that risk, many dermatologists recommend prophylactic removal of these surgically around puberty. This is controversial as bcc is not life threatening, some would suggest excision only when skin cancer transformation is observed.
Excision. There is no hurry. The changes once thought to be skin cancers in these are now recognized to be benign. Expect the lesion to grow during puberty. As the child grows older it is easier to get her understanding & cooperation.
Usually Excision. Since there is a small but significant rate of conversion of nevus sebaceous to various skin cancers, it is usually recommended that they be removed.