How to transfer my old iPad to my son

Here you go. While the Support Article shows an iPhone, the procedure is the same for iPad.

How to factory reset your iPhone, iPad, or iPod touch - Apple Support

Go to Settings > General > Transfer or Reset iPad > Erase All Content and Settings. You will most likely need to enter your credentials followed by a confirmation that you want to erase your iPad.

After you reset all settings, you can create an iCloud account for your son. I have them for my 4 and 7 year old. There are a bunch of granular parental controls you can do from there. A good one is requiring parental approval for any app download whether free or paid, loading money to their Apple Cash, as well as screen time control. Additionally, you can set up a lot of filters for explicit things like mature language or website filtering.