Not safe for kids under 18
Calculator# appears on your phone’s home screen as a calculator app, but is actually a password-protected secret storage for photos, videos, contacts, notes, passwords, and more.

In-depth breakdown

Minimum Age Requirement
There is no age requirement to download the app. It is rated for users 17+.
Messaging Capabillities
Location Sharing
Parental Controls
None, though Apple and Android parental controls can be used at the device level.
The purpose of apps like Calculator# is to create a secret, secure vault for storing sensitive or private information on a mobile device. While there may be good reasons for adults to use an app of this type, it is not recommended for use by children.