What do I need to show when re-entering Thailand after canceling my Non-B visa and work permit?

Once you've cancelled your work permit and both the extensions, might be worth considering a flight to Kuala Lumpur and back. There's a morning flight from Phuket (8am ish I believe? And one that returns at around 18:00, Have a day in KL then fly back to Phuket. It's going to be a similar price to the land border run but, in my opinion anyway, better than sitting in a van all day.

Just so you understand that your work permit isn't what lets you stay here 😕

I am doubtful either of you are on Non-Immigrant type visas anymore and I'd wager you're both on "extensions of stays" from the immigration office (yours based on employment and your son's based on being the child of a foreigner who legally works here)

If your extension runs out in April you'd get termination paperwork from your employer AND proof you cancelled the work permit 🙂 Both you and your child would go to the immigration office where the extension was issued and cancel them. After that you two could bounce out/back to get a 60 day visa exempt entry.

The Ranong border has been known to require you showing proof you're leaving the country within the days you will be stamped in for BUT in your situation you will have that so should be fine.

Keep in mind if you are on an extension of stay (look at the stamp in your passport) you need to cancel it at the immigration office because more than a few land borders won't let people out on "open" (uncancelled) extensions.