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.