On iPhone this splits cleanly into SMS and iMessage, and only one of them is realistically trackable.
SMS — green-bubble messages — can be read by a monitoring product that reads the Messages database, but only with device-level access, which iOS does not grant to App Store apps. In practice, the routes are: the child's iCloud backup (restore to a spare device), or a jailbroken device, which is not something to do to a phone you want to keep secure.
iMessage — blue bubbles — is end-to-end encrypted. Apple holds the keys, and there is no API for third parties. The only access path is the Apple Account: with the account credentials and Messages in iCloud enabled, the messages exist in that account and can be read via a restore. Every "iMessage tracker" product is doing exactly this and wrapping it in a dashboard.
What you can see without any of that: Screen Time's Communication Limits show who your child messages and when, and let you restrict contacts by time of day. Find My gives location. Neither shows content, and for most parents that combination is enough to catch the situations that matter.
If content is truly required, the honest options are: (1) the child's Apple Account plus iCloud backup, with their knowledge; (2) move the child's line to an Android device you administer, where SMS and messaging are readable by an installed agent — Ultimate Phone Spy covers that plus Windows and Mac (sms tracker, system requirements); or (3) accept metadata and limits instead of content.
Anyone selling live iMessage reading on a stock iPhone is selling something that does not exist. That is the single most reliable way to spot a scam in this category.