What about using a summertime kit with water guns and colorizing the water splats? You might have to piece it together from a couple of kits, maybe one with the guns and another with balls you could shrink down to look like GellyBalls?
There are some balls in this kit that could work, and you could recolor them to match your photos:
The gumballs in this kit (look at the top behind the alphabet) could work well as Gellyballs:
Hope this helps!