how do i stop my child from switching VPN to bypass my security

If you are using OurPact to block Apps a VPN App will do nothing even if it could be installed. As it doesn't magically allow access to Apps.

A VPN would only work at the router level. A VPN hides the IP addresses of the websites they want to open from the router, but not from the Website restrictions you can impose on the device. And would be pointless if they cannot open the App to browse the web.

Settings->General->Restrictions->Websites, You can specify which websites are allowed.

There's already plenty of control, learning what things can and can't do is also important.

There is a very simple way to keep your child 'safe' if they insist on attempting to bypass your rules. Take the device away and ground them.

Technology can't substitute for actual parenting.

OurPact is well-defined by its name. It is a service that supports an agreement between parent and child to abide by a set of rules. It is not a way to unilaterally impose security on a child's phone, and was never intended for that purpose.

You don't need an app for VPN; it is built into the phone. However, to use it you need a VPN service provider. They are not free. Besides, VPN services do not let you bypass any of the parental controls that are built into the phone.

Make sure you have Family Sharing set up and have it set that you have to approve App downloads.

Family Sharing - Apple Support