Please be aware that the musically app merged with an app called Tik Tok in August 2018 and the Musical.ly users are being transferred to the new Tik Tok app. Please be aware that Tik Tok requires that users be at least 13 years old to use the app and that anyone under the age of 18 must have approval of a parent or guardian.
Here are two links that give further information regarding these apps.
https://www.internetmatters.org/hub/expert-opinion/musical-ly-app-parents-need-know/