Please double-left-click on the Diagram box for either of the couple, i.e. the father or the mother.
That will display their Property Box with any Children listed in the pane at the bottom below the Spouses.
If there are multiple Spouse: tabs then select each tab in turn, including any that say <no spouse>.
Are you sure that none of those Spouse tabs has any Children listed?
Additionally, double-left-click the Diagram box of the child to show their Property Box details.
The large blue up arrow at the top of the Property Box will link to the parents.
If that still does not explain where the child exists then please describe the characteristics of the three Diagram boxes for the couple and the child, e.g. the background colour, the box outline, the connecting lines, etc.
Alternatively, post a screenshot of the Diagram showing the couple & child with one of their Property Boxes open.