Under their Children, press What are you trying to fix? and choose the option saying a child's relationship to them is marked incorrectly. Blended families are our speciality! Choose step, adopted, foster, guardian, raised by or birth. For example, if your wife's daughter Kate shows as your birth daughter, mark yourself as her step-dad and she keeps her place beside him.
Where to find it
Person page (Safe edit on) → under Parents, Partners or Children → What are you trying to fix?
Only people with the "Add people" and "Remove" ticks, with Safe edit on.
Still stuck? Talk to a real person: send us a message, or email support@mywholefamilytree.com