Hi Jimmy King, I can imagine how frustrating this must be you set up PIN for convenience but Windows still throws admin prompts and slows you down.
Go to Microsoft privacy settings and allow multiplayer and cross-network play make sure your childs account is not blocked by family restrictions sign in with that same account in the Minecraft Launcher then test by adding a server in Multiplayer if it still says disabled recheck the privacy settings.
When your son clicks on Multiplayer in Minecraft does he see the servers list but gets a warning like Multiplayer is disabled or is the button completely grayed out?
Best regards
Lucky