The V1 Nodes get children API "includeSource" feature returns data on the source node e.g. properties, aspects etc. But if you say request additional data such as "allowableOperations" for the list that is returned correctly for the children, but the sourceNode is not augmented with the same extra information.
This means an extra call is needed to retrieve say "allowableOperations" for that source node. This is important in most application use cases where you would have, for example, a Create Folder or Create Content action on the source node (i.e. "folder you are currently in") that you need permission evaluations for.