In your research, you have probably come across a couple that had no children. Unless you mark them as 'having no children' you may come back to them in the future, trying to identify their children.
Legacy Family Tree makes it easy to designate that a couple had no children. In the top left of the Marriage Information screen, just check the box next to "This Couple Had No Children."
If this option is checked, then in the Family View where children are normally listed, it will say in red letters:
** Had No Children **
Be sure that in your sources, or in your research notes, you make some indication of how you proved that they had no children.