# Get selected Node
selection = hou.selectedNodes()

# Loop for each selected node
for node in selection:
    nodeName = node.name() # Get the selected node name
    allChildren = node.allSubChildren() # List all children inside the node
    count = len(allChildren) # Counting the number of node inside
    
    # Loop for each node inside the selected node
    for i in range(0, count):
        childName = allChildren[i].name() # Get the name of the child Node
        newName = nodeName + '_' + childName # Create new name
        allChildren[i].setName(newName) # apply new name
        #print newName