Ah thanks a lot !
I'm not familiar the parent / child concept , I will study it
The second example works without exceptions, apparently the number of WorldMapContainers & tiles are the same so we can do that to get the total number in the map:
Lua Code:
local numHorizontalTiles, numVerticalTiles = GetMapNumTiles()
local numWorldMapContainers = numHorizontalTiles * numVerticalTiles