gelatin
I found a note the other day, that said Tonic water glows under black light. A quick check at Wikipedia, shows that it's the Quinine that flouresces. B2 also flouresces. Are there any other edible substances that flouresce?
We made some Jello with Tonic Water, and while the kids found it inedible, it did look cool under black light. (Hint: use light coloured jello over dark colored to see this better.) Curious if there's a way to make glowing jello that tastes decent.
The only trick I know is to let it start to firm up before mixing in the fruit. You can pour a layer, let it slightly firm up (it'll be kinda a thick goo), add the fruit and the rest of the mix.
... but you don't want to let it set up completely; then you'll just have two layers that haven't bonded well with fruit stuck in there.
Exact time for it to gel depends on what temperature you're resting it, and the size of the container you're chilling it in (mostly it's a matter of surface to mass ratio)
What you are looking for is spherification. You need to use a different hydrocoloid than gelatin. There are a couple of techniques you can use.
If you want solid spheres, you can mix your liquid with agar agar, which is readily available in the asain section of the grocery store, bring it to a simmer, and then use an eye dropper to drop the liquid in to a very cold olive oil bath (put it in the freezer). The dropping action will give you spheres.
You can also use a mixture of sodium alginate and calcium chloride, but those are harder to find and will need to be ordered online. The advantage is that you can make a sphere with a liquid center that "pops" just like real caviar. You mix the alginate in to your liquid and then mix the calcium in to a bath. You use the same eye dropper technique to get spheres. When the alginate and calcium touch, they instantly form a gel. This holds the sphere together. Leaving it for a couple seconds makes it thick enough to hold up.
All the info you need is available in this free recipe guide from Khymos.
I've actually done exactly this dessert before. You can see some of my lessons and issues in another question here. I did it a second time and opted to use a gelled peach (using agar agar) in the center for more of a maki presentation. Enjoy.
Best Answer
I've heard that you can extract oils from leafy greens and once concentrated will provide a reddish tint/glow under a black light. The problem is it may not be any more palatable over gelatin. Your best bet may be to go with mint, but I'm sure most aromatic herbs would work.
Start by blanching a couple of handfuls your chosen greens for a few minutes followed by an ice water bath. Once cooled dry the greens and blend with just barely enough light vegetable oil to make it bind. Think pesto. Keep blending, your goal here is to really break up the cell walls so that the greens will release their oils. Press and strain the mixture retaining the herb oil. Use as appropriate. Maybe mixed into a whipped cream topping for the jello.
It may also be possible to create a tincture of those herbs with a clear alcohol that will glow, but obviously you don't want to give that to the kids either.
Beyond that overripe bananas give off a slight blue glow, but probably not enough to be remarkable.