WordPress multisite subdomains or suddirectories? I’m not sure if you’re getting the two confused. Multisite subdomains are basically fake subdomains that simply have a CNAME or A-Name record added in the DNS records.
That said, and I believe one responder said this, you just need a sitemap for the root domain, and then you’ll have a robots.txt that instructs the major search engine bots to your subdomain.
For instance, I have this in a multisite WordPress network, in my root robots.txt:
User-agent: *
Disallow: /wp-content/plugins/
Sitemap: https://nationalcdp.org/sitemapindex.xml
Sitemap: https://cases.nationalcdp.org/sitemapindex.xml
Sitemap: https://directory.nationalcdp.org/sitemapindex.xml
Sitemap: https://discuss.nationalcdp.org/sitemapindex.xml
Sitemap: https://forums.nationalcdp.org/sitemapindex.xml
Sitemap: https://juntunen.nationalcdp.org/sitemapindex.xml
Sitemap: https://kinsel.nationalcdp.org/sitemapindex.xml
Sitemap: https://johnson.nationalcdp.org/sitemapindex.xml
Sitemap: https://bisbee.nationalcdp.org/sitemapindex.xml
When going to Google/Bing Webmaster tools, I simply add the root directory, add the root sitemap, and let the search engine bots do the rest of the work.
Within your sitemap plugin, be sure that it pings search engines too (which most sitemap plugins do by default), so they’re continually updated.