I just looked at the help for that error and it clearly says that numeric-only keys (even if they are converted to strings) are not valid - you need to append an character too, eg:
Code:
.Nodes.Add , , "P" & CStr(lngNodeParent), strNodeParent
...
.Nodes.Add "P" & CStr(lngNodeParent), tvwChild, "C" & CStr(lngNodeChild), strNodeChild