If I spoke to my daughter like that, she's ask me if she had suddenly woken up in a cult. "We don't do things like this in this house."??? Do you actually speak to your children like this, implying that because you are generous enough to put a roof over their head you can dictate every aspect of their behaviour? So strange and creepy.
I prefer to talk to my daughter like she is a sentient being capable of reason. If I don't want her to do something, I therefore explain why I think it's a bad idea.
OP, there is nothing remotely abnormal about your child wanting a Halloween costume. Cats are a pretty standard choice for Halloween.
If your daughter says she wants to dress up as a cat full time that's an entirely different issue, and obviously that would not be a good idea and if she genuinely can't grasp why (surprising if a 10 year old can't) you need to explain to her, obviously. But as far as I can tell from your OP, she hasn't mentioned this even once. She just asked for a Halloween outfit, and for some reason you are getting your knickers in a twist about a 10 year old asking for a perfectly normal qbe age appropriate thing.
And if you don't buy her one, she can go to Poundland and get one for £2 or something.
Seriously, this thread is batshit - I can't understand how any of this is a problem.....