I know I should have said something but just couldn't find the words

[Oh Joy]

Maybe I'm twisted, but I see the follow-up phone call as X attempting her best manners.  She called - without needing anything from the OP - to both thank and apologize to her, and I read the 'she shouldn't have to...' as more of a clarification of a misunderstanding.

Even if X was wrong in her perception of what's appropriate (dropping in, needing to ask family), I give credit for clumsily attempting to do the right thing in follow-up.

Or I'm just reading the whole situation incorrectly.   

[BeagleMommy]

The child saying "If I have to ask for it, I don't want it" is code for "I'm not getting my way so I'm going to pout and make you feel guilty".

OP, your silence was deafening. 

[*inviteseller]

I think it should be a felony for people to pull the 'faaaammiiiiillllllyyyyy" card to justify rude actions or get their own way!  Silence is the only way to go here.  I would be so taken aback that after a 2 year absence from each others lives, that she found your house instead of a gas station or restaurant for the kids bathroom break.   

[EllenS]

Personally, I see Y saying "if I have to ask for it, I don't want it" as being a flag she is being raised by PA and manipulative people, and it is her way of refusing to play along. 

If she was old enough to have that conversation, she was probably mortified to be put in such a wierd position and was trying to figure out what the boundaries are.  I mean, really.  Can you imagine, as a kid, being taken to pee at the house of a total stranger you've been *told* is related to you? 
 Not knowing if you are welcome or not?

And then aggressively scolded in front of said stranger?  Poor kid, she was probably expecting to stop at a McDonalds or gas station.  Did X even make proper introductions?  Poor Y.

[Redneck Gravy]

Let's see if I have this correct:

1. Having not seen X in two years, she drops by unannounced and asks you to allow her child to use your bathroom...   

2. Corrects Y in front of you about taking candy without asking to which Y makes a snarky reply,  (to which my reply would have been along the lines of "your choice")

3. Calls you to thank you again for allowing them to drop in and use your facilities and THEN corrects you about how you should have allowed Y to eat a treat without asking!

I can't imagine that anything other than complete silence would have been the polite response!  I probably would have opened the door and said "sorry you have to be on your way, have a good evening."

Just wow, still all these months later.

[CrochetFanatic]

Personally, I see Y saying "if I have to ask for it, I don't want it" as being a flag she is being raised by PA and manipulative people, and it is her way of refusing to play along. 

If she was old enough to have that conversation, she was probably mortified to be put in such a wierd position and was trying to figure out what the boundaries are.  I mean, really.  Can you imagine, as a kid, being taken to pee at the house of a total stranger you've been *told* is related to you? 
 Not knowing if you are welcome or not?

And then aggressively scolded in front of said stranger?  Poor kid, she was probably expecting to stop at a McDonalds or gas station.  Did X even make proper introductions?  Poor Y.

This.  Now, don't misunderstand me.  It was rude of Y to do what she did, and it was rude of her grandmother to correct you over the phone.  To be honest, that action made my jaw drop more than Y's did.  On the other hand, it sounds like she (Y) isn't being taught any better and is receiving mixed messages.  "Yeah, we're family.  Hey, get out of that candy dish!"  "What'd I do?"  At that age, I probably would have gotten defensive too.  Her feelings were probably hurt.  That's no excuse, of course.  But if she's not learning about boundaries now, she may be in for a lot of grief later on.

I think your response of complete silence was perfect.  Even if the kids don't know right from wrong, the adults certainly should!

[Pen^2]

I agree with what's already been said--silence is deafening enough to someone who listens. Someone who doesn't isn't worth much time.

We can make all kinds of guesses about Y and the whole situation there, but in the end it's not particularly relevant. It only stands that Y was rude for whatever reason, and X tried to explain it away afterwards. X may have been trying to apologise (poorly) or she might have been being snide; either way, the initial response would be the same.

The only other response to X's statement I can think of would be, "what an interesting assumption" or a variation thereupon. But who can possibly be expected to be ready to say something so unexpected and ridiculous.

Or... this really is for only the best actors among, but... I know someone who's brother once said something to her like, "you're my sister, so you have to do *enormous big favour* for me whenever I want," and she just burst into laughter at what he'd said, leaving him dumbstruck. There's nothing like making someone realise how foolish what they've said is to get the point across. But I don't think most people could ever pull this off.