korma is right,
You may not even need the user to update the list manually if the goal is to create a navigation menu. All you need in that case is Wayfinder, which is one of MODx’s most valuable assets. The list would be automatically updated each time you create a new child document under services.
If however your list is used within the content section, adding a new child is as easy as placing your cursor at the end of the last element and hitting enter. Just like you would do in MS Word. It doesn’t get any easier than that.
With more details, I’m sure there’ll be better help.
Hope this helps
A MODx Fanatic