The Japanese term for this phenomenon is 「声の高低(こえのこうてい)」 (koe no kōtei), which refers to variations in pitch. More specifically, the tendency to use a higher-pitched voice in certain social contexts—often associated with politeness, deference, or perceived cuteness—is sometimes called 「声のトーン」 (koe no tōn, "voice tone") or 「ぶりっ子声」 (burikko-goe, referring to an intentionally cute or childish voice). If you're referring to the deliberate use of a higher-pitched voice to sound more polite or endearing, the term 「作った声」 (tsukutta koe, "constructed/artificial voice") may also apply.