Few things:

1) yes, it is, because JKR declared it so

2) Welcome to the HP wiki! I'm CHT, and below are some useful links!

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If you need help, contact me or my fellow dmods (Sir memes a lot more than needed, TheHacktivist42 and Jimbob Daddario directioner Herondale-Blackthorn) or discussion-focused admin Reverb frost.

Have fun!

3) I deleted one of your other posts because it was about a ship between an adult and a student, which is against our Guidelines- please don’t continue to discuss similar ships

CHT64 my apologies for posting something that violated community guidelines. Just out of curiosity, what post are you referring to?

It was the poll you made about Harry/Voldemort

Rowling herself said it was, so like it or not, it's canon.

Once i mentioned to my friend that doesnt think cursed child is cannon that the authour said it is cannon. she turns around and says to me, 'do you believe everything the authour says is true?'. I must say that is a good way to win an arguement.

(BTW if this is against guidelines or offensive to anyone, sorry)

Sadly yes

@RAVENCLAWGAL09 "Do you believe everything the author says is true?"

Heck nah!

Rowling herself said it's canon. So whether you like it or not, whether you accept it or not, it's canon.