Definitely heard and sang it.
True, but John raised Sean, and adopted him. If you ever hear Sean Astin refer to “my father,” he’s talking about John.
OK, I have now seen “The Addams Family in Court” and Gomez does indeed bring Thing to the courtroom in a briefcase. So, yes, in the 1960s show Thing is a hand. That being said…
The original Charles Addams cartoons with the family sometimes have a person peeking behind or out from somewhere, like the railing here. This may or may not be the Thing (which, to my knowledge, was only ever directly mentioned in the sign seen in this cartoon). Other possibilities include the hands seen here (more like the show) or whatever is in the carrier.
Unfortunately, ‘this cartoon’ is not displayed. 
It looks like Kilroy.
In the one with the carrier, are they standing in line to buy airline tickets? 
It works for me. Here it is from another source.
From a British perspective, that’s not Kilroy, it’s Mr Chad – a graffiti format that predated the US entry into the war as a commentary on wartime shortages. The slogan that accompanied the character was variations on “Wot? No bacon?” (Or whatever commodity was currently in short supply).
“Kilroy was here” was originally just that text: it seems that at some point GIs merged the two formats, but in the UK they remained separate.
Aha! Yes, I remember that one. (I need to pull out my books more often.)
That’s how I remember it, when I ‘researched’ it a couple of decades ago. Thank you for spreading the word. 
Sorry, I didn’t mean to imply otherwise. But from what I have read, Sean maintained a good relationship with all of his “dads.” Anyway, I was mostly referring to the lack of a family resemblance between him and John Astin.
For my part, I was not intending to correct or contradict you; merely trying to add some information that might be interesting. Sean Astin’s true parentage was a matter of confusion for some time. But like you, I understand that he is on good terms with all three of his various father figures.