How do I stop my child from hacking the parental controls?

Parental controls are useful to keep parents from making changes to settings, or otherwise causing the kids to need to have to fix problems for their parents.

Parental controls for controlling kids’ access, not so much. Parental controls as a means to keep kids from making changes is usually a losing battle, to be blunt. Kids are way better at learning and bypassing security than parents are at establishing and maintaining it, and kids are masters at “shoulder surfing” to acquire passwords, and at just observing and learning their parents’ habits, choices, and norms. This as applied to passwords and password choices, or otherwise.

If you do manage to lock down the environment and the network and can then maintain that lock (the far tougher part), the kids will usually then bypass all of your blocks. That’s entirely feasible on a kid’s budget, too. Which means you’ll have no insight into what’s going on, and what the kids are accessing. If they don’t already know how to do that bypass, they’ll be able to find out from other kids that do if they’re at all inclined to learn.

If you really want to increase your own security, and learn more about how to harden your kids’ security while you’re at it, start here: Device and Data Access when Personal Safety is At Risk. All of that won’t be applicable here, but you can likely learn a few things from that. Enabling two-factor authentication, for instance, if that’s not already enabled here. Your password choices are seemingly suspect at best, too.

As suggested use a strong password for Screen Time.

You cannot disable the Settings App, but you can prevent anyone from accessing Screen Time settings with a strong password, and don't tell your son that password.

They can't get into the Screen Time Restrictions if you have used a strong Screen Time Passcode.