You said you tried to add in the child. How did you try to do this and who was he the child of? You also said the relationship already existed. Is the child attached to the correct parents?
From what you said I suspect this is the child of the person you had attached to the wrong parents. Detaching that person and attaching him to the correct parents has no effect on his relationship to has spouse or to their children. The children were never detached from their parents.
There are a couple of ways to split off a branch of your file.
Go to File, Export, Selected Individuals. This will bring up your index. Select the person you are interested in. If you are only interested in descendants select the Descendants option. If you want ancestors select the Ancestors option. The Ancestors option gives you the option to include descendants of those ancestors. Review all the options on who to include. You can then export the selected people and import them as a new tree.
Select Publish, Charts. Decide which chart includes the people you want and review all the options. Make sure you include enough generations. Right click anywhere on the chart and export the people in the chart.
Both method 1 and method 2 can be used to split off a branch of your file that you want to share with someone.
Go to File, Export and export the entire file and import under a different name, Testfile, or whatever suits you. Now you can experiment to your heart’s content without it.worrying about messing up your real file. Once you are confident with what you are doing go back to your real file and make your changes.
It still pays to take a backup before trying anything complicated.