I would love to try making this cake. The non-exploding version.
Another vote for: This is an amusing idea in theory, but in practice, you should be wearing Bad Idea Jeans.
I am a third who started to read this thread with an inner child in me screaming, “please, please don’t do this.” Wheelz, you are not a killjoy but the first voice of reason and having a prank played on a young girl by an adult is not a prank, to the young girl it will only feel like everyone hates her and is getting a laugh at her expense.
A pie-in-a-face scenario is fine when others are suffering or participating, but for a young girl’s birthday, let her enjoy her day without without being the brunt of a joke.
And don’t forget, this is not a fully-functioning adult you’d be pranking.
To a not-even-yet-a-teen girl, being embarrassed is worse than famine, cancer or nuclear annihilation.
It’s the second-worst thing that could happen.
The worst? Being embarrassed in front of your friends.
Friends that can laugh about you behind your back at school, and tell that snotty Missy Randall who wasn’t invited, and her pompom squad friends will taunt you all the next week.
And these days, somebody’d have it up on YouTube that afternoon.
I have to agree with others that this is a weird and cruel idea. Unless this is done very safely and unless this girl has great self-esteem, I think damage could be done, physically and mentally. She goes from having a fun birthday to cleaning icing out of hair and possibly eyes.
If you just have to play a gag, how about those self-relighting candles?
c4
Seriously, she’s 12? This seems cruel.
For what it is worth, to me it sounds like you can’t handle being on the sidelines while a little girl has a party that is for her. Do you feel a need to upstage it with your dramatic gag so that all anyone will remember about her birthday event is you you you?
What a cruel and sadistic idea.
Actually, given that the OP apparently has something of a reputation, how about this? Do nothing unusual about the cake. Nothing at all. But keep dropping hints about what might happen. Like, tell her to cut the cake, hand her the knife, and then stand way back. Put the cake on an obviously carefully-prepared table, and make sure nobody sees the underside of it. Have cleaning supplies conspicuously closely available. Do the whole setup outside, or in a basement, or someplace else it’d be easier to clean up a mess, and firmly insist that she really doesn’t want to move it to the dining room first. Tell others “hey, watch this”, and make sure you have a camera running at the time. And do all of this with a wicked gleam in your eye.
Really dude, this is a stupid idea. Drop it and move on.
That’s an awesome way to fast track yourself to creepy-Uncle-ville.
Is there a specific reason you want to do this to her? Is she a brat or something? (Not that it would make it okay)
Why not just have Justin Bieber jump out of the cake for her?
I appreciate the concern but you guys don’t know my nieces. I’m much more worried that the other niece will be mad that her cake didn’t explode. Trust me to know my nieces–they will want an exploding cake for the next 5 years. I’m the uncle who throws them slime parties every year which they love. Last year they smashed sardines into each other’s hair.
(I am happy to say that neither of my nieces like Justin Bieber. It must be because I played The Ramones to put them asleep when they were babies.)
The two ideas I like so far are:
The catapult. What about sandwiching a mouse-trap between two pieces of cardboard? I can probably rig something up to trigger the trap, which would send the top piece of cardboard flying. The trap will be glued to the bottom to keep it from moving. Jello and whipped cream might make a good projectile. The biggest issue I can see is the myriad of ways that trap could do real damage.
Air pressure. I might be able to buy one of those cheap toys that have an air bladder inside them attached to a pump. I could sandwich that between two pieces of cardboard as well. The main problem is concealing the pump and the hose, not to mention how to trigger it.
What do you think of those?
I’d skip the directional part and make a ‘cake’ by coiling several of those balloons used for balloon animals into a cake shape and then frosting that with whipped cream or some other light icing.
Make sure you have inflated the balloons to the maximum point you can before they would burst normally.
Hand nieceling a sharp knife and get out of the blast zone.
ETA: you might also frost it with shaving cream, or at least the ones you make for the trial versions. You will be making several test runs, right?
I like a sort of foot-pump idea. Start with a hollowed cake, filled with whipped cream. Have a footpump under the table, with a tube running to under the cake. Stomp the pump, air powered cream explosion ensues. You could even use a plastic bottle. Tell your niece she needs to wear shades cause the candles are so bright.
Si
I never agreed to this! 
Hmmm…I like this. I could cut off the bottom of a plastic bottle, turn it upside down, and feed the air hose through the bottle mouth. If I fill it with whipped cream it would be semi-directional. I will have to do some testing. =D
Perhaps an oozing or spraying cake? Try rigging a length of tubing to a can of aerosol whipped cream. If you put it into the cake so that the tube is just below the top frosting, it should spray cream. You could aim it wherever you want.
If you put the tubing under the top layer of cake, the cream would ooze out the sides fairly rapidly, I should think.
Post photos 
Si
I hope you plan on keeping another cake in reserve – that way, at least there’ll be an actually cake-cake for everyone to eat.
(Please though, at least check in advance to see that she doesn’t have some friends from school, including a crush coming. Just in case?)