"Understanding the worries" doesn't mean avoiding something that could ever cause them harm though. Parents understand there could be potential dangers to everything- walking to school with friends, going on school trips, sleepovers, learning to drive, playing a sport....the answer is not to isolate your child and wrap them in cotton wool until they're 21.
From your wording I assume your child is much younger. I agree with the pp, why would you actively support your child being upset, left out and bullied. OP is damaging his relationship with his peers and with herself - he literally felt forced into hiding the app as a calculator. How is any of this good for him?
I dont get the logic - you thinknthe apps are bad for kids' mental health but crying everyday and being left out and bullied is superior? The apps endanger kids' safety so its better to foster a home environment where your child feels they have to lie and deceive you?
To me, just saying a blanket no is lazy parenting. Rather than making the effort to talk to them about what they see, research ways of restricting time on the apps, enforce turning phone off/handing it over at bedtime, doing spot checks etc you just say no, leaving your child so miserable he cries everyday.