iPad mini was reset for my son and now it won't connect to any wifi networks at all

Assuming that the iPad was working with same WiFi network(s) prior its reset, whilst perhaps counterintuitive, a restart of your WiFi Router may resolve the problem…

To restart you home router, disconnect power - then wait at least 30 seconds before restarting. When you router has rebooted (which may take several minutes), force-restart your iPad:

Force restart iPad (models with Face ID or with Touch ID in the top button)

If your iPad doesn’t have the Home button, do the following:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. When the Apple logo appears, release the top button.

Force restart iPad (models with the Home button)

Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.

Your last option is to perform a Factory Restore - and do not restore from a backup. This will either resolve the issue, or if unsuccessful, confirm a hardware failure:

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

Try a reset of your Network Settings:

iPadOS16

Settings > General > Transfer or Reset iPad > Reset > Reset Network Settings

iPadOS15 or earlier

Settings > General > Reset > Reset Network Settings

Some mesh routers don’t work particularly well with Apple devices. You may need to refine the router settings.