Bumping this because I want to know if the OP went through with this, and if so, what happened?
Her birthday is not until December. I’ve got the parts but I haven’t tested it out yet. I need to do that soon.
Special Effects guy checking in.
You will need the following, which you must conceal under the tablecloth, preferably bolted to the bottom of the table. A small pancake type air compressor, some various valves and tubing and a strong balloon and pipe clamp. Drill a small hole in the table top and run the tubing out of it. Using the hose clamp, attach a strong giant deflated balloon to the tubing. Run the rest of it to the compressor and have a quick release valve in the line somewhere. Attach some monofiliment line to the valve. Cut a hole in your platter and in your cake. Place the assembled cake and platter over the hardware, being certain to feed the balloon into the hole in the bottom of the cake.
Now at the last moment pretend to drop something important, like your camera, and when you bend down to retrieve it, grab the mono line and be prepared to yank the release at the last moment. Double up the fun, and the safety by having her wear goggles and running her a line about the cake doing something else strange.
Alternatively, you could run the compressor to a screen door cylinder mounted under the platter. When engaged it will launch the cake off the table. 
If you like this idea, search some haunted house forums for examples of how to assemble this device. It’s quite simple.
Hopefully you won’t get sued by her friends parents if your prank goes wrong and they get hurt. Not saying they will but who knows. It’s something to consider although I don’t know what type of cake you are making. It still seems like a bad idea.
Uh, thanks. I think. 
It’s whipped cream! Sheesh.
heh. Sorry, I forget not everyone is technically inclined.
Think of it like legos.
You need an adapter from the compressor to the valve, from valve to hose, and the hose to the cylinder/ ballooon. The cylinder has that little arm that pushes out when it is filled with air. So if you connect it all up, when you pull the valve, it fills with air, and the little arm launches the cake. Whoosh! If you use the balloon, it fills with air fast and explodes the cake.
It’s just that drilling a hole in the table top is a little more involved than I’d like, especially since it’s not my table. 
I think it sounds like a great idea.
An easy way to hide the device would be to build a plywood box to present the cake on. It won’t look funny if you decorate it and you can mount everything underneath it. I’d suggest trying to rig it to be remote controlled so it will look less obvious.
Good idea. Or do it outside on one of those tables that has an umbrella hole.
Get some anti-matter (available in better equipped joke stores)…a few ounces should be just fine…
Imagine the look on her face when the exploding cake removes the atmosphere from that side of the planet…
ahhh, its fun being an uncle…
I don’t know about special effects, but I do know about knives. Your niece having a knife that is sharp and/or pointy in her hands as she is startled is a bad idea. People have all sorts of different reactions to being startled that may range from just blinking or flinching on up to screaming and throwing what ever is their hands. Parents of other guests might not sue over a little whipped cream, but when their little snowflake gets struck by a sharp object, they’re not quite so forgiving.
Sure, all over her party-best clothes. And your sister’s walls and floor. Please don’t do this.
Brilliant! Outside, umbrella hole, garden hose, voila!
Ants.
Flies.
Roaches.
Just…no.:smack::dubious:
Surely this is a joke? Lot’s of embarrassing things happen when kids are younger. Sure, they may be remembered throughout high school, but it’s not going to matter for very long. And boys, which are the requirement to get heirs, aren’t going to care much at all.
And, then, after high school, you aren’t going to be with the same people anyways.
I think that the people who are worried about scarring the young lady are some really strange people.
Outside of your turning her into some Lizzie Borden, remember to vid it and post it on Youtube.
best wishes,
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Well, I made the apparatus and it works pretty well. I used a rubber bladder that photographers use for blowing dust off their negatives. Unfortunately it doesn’t look like it’ll get used–the current plan is to celebrate her birthday at a restaurant and it’s not worth the hassle to try and set it up. I’ll have to save it for the other niece.
Have the waiter cut it for her. That way everybody wins - she won’t be traumatized, her parents won’t sue, and you won’t have to clean it up!
Obviously record, post on youtube and provide link.
How old is the other girl?
That sounds like a teenage boy.