I'm going to ask my 9 year old brother not to spoil a movie I haven't seen yet
Why do you want him to keep it a secret?
Because I do not want him to spoil the movie before I get a chance to see it.
It makes sense to not want someone to spoil a movie you look forward to watching. Just ask in a polite and respectful manner so as to prevent them spoiling things anyways just to spite you.
Maybe my tone will change over text, but I'm going to be nice about it.
That is great. It can be tough to monitor your tone when sending text messages sometimes, so if you have time to call this may be an even safer option.