Paul -- on profiles you manage (or where you are on the Trusted List), on the profile view (not edit mode) there are those little links to add spouse (one only - more than one can only be done in edit mode), add parent, add child, add sibling.
If you go into edit mode and click the no more children option, it will remove the link from the profile view of that parent. You will need to do the same for the other parent. (You can still add children from the edit mode, even with the no more children marked, but the link on profile view will no longer be there.)
Similarly the option to add a sibling from the profile view screen can be removed by going into edit mode and clicking the no more siblings option. This will need to be done for each child of the parents.
It can be tedious when there are lots of children, but it's the "safer" option, especially when a parent may have multiple spouses with children by them. Having something automatically toggle "no more siblings" when the mother is marked "no more children" might be incorrect if the father has more children by another wife - and vice versa.
Most of those who do this seem to do it for the "tidier look" it gives a profile. (It's one of my reasons.) Some don't like to for the reasons Dennis has given. Sometimes a child previously unknown is found later. (But, as explained, even with the no more children toggled off, you can still add them in edit mode.)
edited to add missing word
edited to correct an errant 4
