If you use Family Tree Builder, read the following article: How do I detach a child from its family in Family Tree Builder?
It is now possible to remove a child from its family in your online family tree.
You can do so using the ‘Remove Connection’ feature.
You will then be able to attach the child to another existing family in your tree, using the ‘Connect to existing person’ feature.
Please see the following FAQ post about using these features: How do I use the online Connect / Remove Connection feature?