You can't delete profiles on WikiTree. If this child didn't exist, I would suggest that you change her first name and dob to one of the other children/siblings and merge them, eliminating her profile altogether.
If she did exist, but has different parents, then click her EDIT tab across the top of her profile, scroll on right side to Father - click the edit/delete and on the next page, scroll to the bottom and choose Remove Father. Then do the same for the incorrect mother.