Whats the best one for monitoring your child?

There are none that won’t put your child at more risk than anything that you might detect. See→Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

Apple goes to extraordinary lengths to assure that no one can monitor what you have on your phone. So the only way to actually spy on an iPhone is to hack ("jailbreak") it. But once hacked most Apple security is disabled, so, while you can monitor it, so can predators and other criminals.