I'm still not buying this idea that kids are too 'tech savvy' and this defeats the purpose of checking (not spying) on their phone.
Yes, kids are more 'au fait' with phones etc. because they've be brought up with them in the household but they're still kids, not Mi5 agents. If you think your child is hiding something (change of mood, excessive secrecy etc) then educate yourself.
I mentioned in a previous post that I'm an IT professional (digital marketing mind you, not security) so, before anyone says "it's easy for you", I simply Googled:
"Apps kids use to hide photos" and was given this link
www.carolinaparent.com/CP/Secret-Apps-Your-Teen-May-Be-Using-to-Hide-Photos-Videos-Messages-and-Files/
Probably not exhaustive but a good start and it took me 20 seconds to find (it even includes the, distinctive, secret calculator app a previous poster mentioned).
"But, just because they have the app, I can't see what's inside without a password"...Well tell them to open up the app, show you what they're hiding and then delete it...YOU are the parent (who is invariably paying for their phone) YOU are in charge.
So, what about their privacy? As another poster mentioned, we are not looking to police their lives. If I have to have words with my Stepdaughter and she then texts her friend "I hate my stepdad, he's such a cock! I'm glad he's not my real dad" or something, yes, I wouldn't like it, I wouldn't approve of the language and, had she said it to my face, she's have been picked up on it. However, it's typical teenage behaviour and I'm not going to hold her to account as it's not a danger (which is what we're looking for).
IF she said, for example, "I hate my Stepdad, think I'll run away and stay with Colin tonight" (contrived but feasible) then you have identified a potential danger which then SHOULD be addressed.