Do I need to cancel my ED visa and guardian visa with immigration before leaving Thailand for good?

If the ED Visa is an extension to stay 365 days, you need to cancel it prior leaving.

I'm sure if you looked at your current stamp or your child's you would see that it clearly says "extension of stay permitted until" and has a date.

Those usually need to be cancelled with documentation by the organization that supported them (in your case the school) before you leave so as not to cause you any issue down the road should you try to get another long term visa/extension

BUT

in your case you're leaving for good, as in you're not going to come back here and try to get a long term visa/extension. You said you're only going to return as "tourists" on either free stamp or tourist visa entries. Neither of those will be affected by you leaving without cancelling the current extensions you're on. Now should you follow the procedure? In theory yes, but in reality nothing with happen to you or your child if you don't.

So the choice is yours. .. .

Should be cancelled with a letter from the school.

Guessing he is on an extension as opposed to the initial visa. You should cancel that, but if you never intend to return, absolutely no problem.