This depends on the exact kind of family relationship. If you just want to have the kid disappear from the tree, you can click on the kid, choose "More Actions" and then select "Delete [name of child]". (If the child has children themselves and/or a partner that is/are not connected to other people on the tree, you'll have to delete them first.)
If you want to keep the child but change their parents, you have to click on the child, select "More Actions" again and then "Set Parents". The kid's current parents will be displayed as "Primary Parents", and if you click on their names, you'll be able to select which people on the tree should be the kid's parents instead. If the child has (ex-)partners or children, they'll be transfered as well; siblings won't, however, so if you want to change multiple children's parents, you'll need to do this for every child.
Hope this helps!
