round teeth
A wide, open-mouthed smile where the front upper teeth are drawn as a single round semicircle.
Often, it is reserved for depictions of youthfulness, playful personality, and innocence, and is very often a recurring physical trait in certain characters (as with Mizuno Kaede and Aoi Sora). However, it is common enough that it might show up without any qualifiers.
Differs from buck teeth in that buck teeth describes only the two upper front teeth, or a something that resembles just those two front teeth, while round teeth describes the entire row of front upper teeth.