It's horrifying to hear that so many grandmothers behave like this. It's never entered my head to arrange my granddaughter's birthday party! I'm not her mum.
OP, I agree with PPs that you really need to tell her yourself. Your husband has grown up with her brainwashing, which is why it never occurred to him that it was not on, and that it's a powerplay, rather than a nice gesture, so he's unlikely to tell her in an unambiguous way, that she's not to do things like this, if he even speaks to her at all.
Most men like this, put off speaking to their mothers, because they know how badly she'll react, and they try to avoid doing anything to upset her. If they do finally speak to her, they'll make it sound like you're the bad guy, and they're on her side.
At the moment, there's plenty of time for her to cancel things, but if you don't say anything, it'll be too late, and you'll be pressured into going along with it.