Well congrats on your new addition Libs.
I personally don't recall anything in canon saying that Dean changed Sam's nappies and it has been mentioned on the show that Pastor Jim took care of them. It WAS when they were older but what's to say it didn't happen when they were much younger too.
Anyway, yes Sam is thoughtless and a big meanie blah blah blah but someone had to ask Tamara and Isaac that question in order to inform the viewer - like I said they use him for plot purposes. I doubt the writers thought: I know we'll make Sam look thoughtless by having him ask this question, it was probably more like: we'll have Sam ask the question so as to inform the viewer.
It's the viewer that forms those opinions about Sam and they don't necessarily have any foundation as far as the writers are concerned. People can invent or interpret stuff all they want but if it wasn't the writers intention then it's all baloney considering they write the show.
I was thinking he could have told them in order to convinve them why they should leave Dean out of the hunting. Sam basically tries to convince them that Dean deserves a normal life, they want to know why he's so special, Sam goes into a little more detail about hell to convince them/gets carried away maybe. Ahhh anyway...
I, for one, never thought less of Sam for blundering into thoes situations. As you say someone HAD to do it, Bobby KNEW them already AND Dean isn't the type to pry.
Still I can see how he could come off as thoughtless and clueless by just blurting out everyone else's business and prying into their personal stuff. Still, he was ALSO quick to 'out' himself when he broke the final seal and before that about being a psychic kid with a demon connection (even at the roadhouse). So, that's just who he is I guess.