How are they truly sure? Of the gender, I mean.
That’s my problem with it, that you can determine biological sex at that point, but that doesn’t say anything about what gender they will eventually gravitate to.
It would suck to end up causing this much damage and then find out your child has a different gender than what you assumed it was, making the whole thing even more pointless. 
“I burned 80,000 acres and you want to what?!!!”
I heard this described as “an accident”. That word is so wrong. We need a word for when someone does some ridiculously stupid shit followed by a not-unforeseeable disastrous result.
If you called it a “Sex Reveal Party” people might show up with condoms, lube, and the wrong idea.
Genital Exposure Party?
Nah. Too easy.
Criminal negligence?
Too long. The only thing I can come up with is “hombée”, which derives from “hold my beer …” but with the implicit connection to “homme flambé”. Never catch on, though.
And unfortunately “Darwin Award candidate” doesn’t apply in this case, since they have reproduced already, and everybody is alive.
Practice run.
Last I heard, there are people in their twenties who are only now revealing their gender.
Some people just aren’t very bright. I was too close to a wildfire started by kids shooting flaming arrows–with dad’s permission–on a tinder-dry mountainside. Actually I was hiking down the mountain when the fire started and then blew up. It got pretty smoky, and ash was blowing all around. I got down 5 minutes before tanker planes started chemical-bombing the place.
There was a gender reveal party–can’t recall where–in which the family accidentally built a pipe bomb. It killed an elderly woman.
Gender reveal parties are ridiculous. I’d like to go to one where the cake is cut to reveal: purple! 
I also think “pushing presents” for new mothers are dumb. Your gift is your baby.
My pet peeve: Using the word “wildfire” to describe a fire set by human action.
If a fire is caused by, e.g., lightning, that’s “wild”. If it’s caused by humans, that’s not “wild”. Maybe it’s “domestic”. But if it gets out of control, then it’s a “feral” fire.
We held your gender reveal party when you were just gleam in your father’s eye!
I hate to set off someone’s pet peeve, so I apologize. But according to Webster’s…
Definition of wildfire
1 : a sweeping and destructive conflagration especially in a wilderness or a rural area
I also checked Britannica and National Geographic. No mention of natural versus human causes.
Can confirm.
She’s pregnant. It’s hardly a sex reveal.
The California state fire authorities CALFIRE use the term “wildland fire” to describe a fire on, well, wild = unimproved land.
Which frees up the word “wildfire” for one with natural causes. But so far they’re not winning the battle of usage.
Smokey the Bear is always telling me “Only YOU can prevent wildfires,*” and has been doing so since I was a kid. Since I can’t possibly prevent lightning strikes or things like that, it seems like “wildfires” has referred to fires caused by human negligence for quite awhile now.
* As a matter of fact, I’m old enough to remember when he talked about “forest fires,” rather than “wildfires.”