Within linguistics, there is a small but respected body of research on these children born to Esperantist parents, and the term "native speaker" is commonly employed (though even for natural languages that term is problematic, so "L1 speaker" might be preferred). Particularly within Europe, the parents are taking the kids to the periodic meetups with other families, so those kids are getting the same amount of exposure to the language within a community as city kids brought back to rural grandparents' house for holidays.

I recently met one

That's exatly what native means.