monitor child's phone

What you are asking for is not technologically feasible, especially if you let your child install any apps they want to install. The parental controls on the phone allow you to prevent installation of apps without your approval. But you still will not be able to monitor their every message other than the old fashioned way of examining the phone periodically or backing it up and using backup extractor software to view what they have used the phone for.

Apple's secure firewalls protect your child. The monitoring software, as you discovered, bypasses Apple's security. That can allow you to monitor them, but it also allows any hackers or predators to monitor them, which is a much greater threat to their safety than any message conversations they may have.

I understand your concern as I am a parent emeritus, but this is a really complex issue. ("For every complex problem, there is a simple solution - and it is wrong" - paraphrase of the original quote but close enough). If you implement a way to spy on your child, how do you keep it limited to that purpose only, and not allow it to be used to spy on an ex, or a soon-to-be ex, or on someone being stalked, or how do you prevent a business or government from spying on people in general? If there was a general solution I could support it, but as an expert in cybersecurity I don't see a way to such limit it to a narrow target. I'm open to ideas, but I haven't seen one yet that would actually work without creating bigger problems. All of the ones currently available for both Apple products and Android phones have the same issues.