Is Naisho no Tsubomi hentai?
No
Rated PG (Children) on MyAnimeList and not tagged as Hentai.

OVA · 2008 · 3 episodes · PG - Children
Romance, School, Shoujo
Tsubomi Tachibana is a fifth grader dealing with rather sensitive issues such as her mother's pregnancy, her first period and the strange feelings she has started to get when she has been around boys. (Source: ANN)
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