The Secret Calculator Plus App is one from the "Vault App" family. The core idea behind the app is to store your photo's and video's away from the "front screen" of the phone you are using.
1) Why do kids use this?
Young people use this app to hide the photo's they don't want their parent or guardians necessarily seeing. Sometimes they're photo's of interests or hobbies that they don't want you to know about, some are pictures of themselves not wearing much or anything at all. They know that even if you took their phone and "checked through it" that you wouldn't find these photo's so they feel relatively at ease then is taking / storing their photo's and video's.
2) How can I tell if it's a vault app?
There are tiny little differences in every vault app that distinguish it from the genuine app, in this one look out for the "+" sign at the end of the word "calculator". At the moment that really is the only difference. Once you go within the app, it will work just like a calculator until you put a four digit number in, preset by the young person, it will then unlock the "back screen" of the app and show you anything stored behind.
3) What should I do if I find this app on my child's phone but I can't see what's behind it?
This is where parenting comes into it's own. It is recommended that you open a conversation with your child about this without making them feel in trouble and showing you are 'just interested'. It's just as likely that the things behind the app are of no real concern to parents as them being a concern.
There is nothing more important than open conversation with your child.
A good tip for any parent / guardian is to check their downloads on the Appstore as this will show "Secret Calculator Plus" as being downloaded.
Remember that you own the phone as a parent 99% of the time. You can take control of a situation if you fear your child is at risk but try to use this as a last resort.