I’m not fully understanding your request here. Do you want to create one file at a time, and make it link to an index file in the parent folder? Or do you want to create all the files in the sub folder, in one go?
How do you foresee locating which file in the parent folder is to be used as the index file? Will it always by named Index.md?
Another question which could affect the outcome is where will you be when wanting to create this/these file(s)? Will you be in the index file, or somewhere else?
In most cases, you might need to extend from pure template functions, and either use the obsidian API, javascript API or dataview API to achieve all of your goals. And based on my limited knowledge of your use case, it seems like there needs to be at least a little javascript involved to achieved your goals.