In Revolution, having children and being a folder are independent. The "Container" checkbox when you edit the resource determines whether it's a container (folder) or not, regardless of whether it has children. The icon used in the tree, however, is based on whether the resource has children, as Bruno17 says.
The URL for a resource depends on its container status (not whether it has children) and the container_suffix system setting.
I think Wayfinder uses the container status of the resource, but I'm not positive.
------------------------------------------------------------------------------------------
PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html