I don't understand the specifics of your problem?
Do you mean you have a child of a person that doesn't show as a child when you've selected a parent in the Family Tab of the People Workspace?
There could be all kinds of things going on. First of all, do you have Blended Families turned off or on (the button on far right of button bar above the list of children).?
If a man had four wives and children by each, each wife had two husbands with children by each, you could have all kinds of situations going on.
The list of children, with blended families off, will show only children that the parent couple that is showing had together.
If you turn blended families on for that couple, it will show ALL children of the two parents showing - with a single dad, single mom, or couple icon showing to show which was parent to who, or if both were parents to the children listed. However, it wouldn't show children another spouse had with another person.
Please post the specifics of your problem. Who was a child of who that doesn't show in the list of children?
If you are talking about an ancestor not showing for a selected person - do you have that person in "preferred" status?