EcoMunch is a predator-prey simulation based on A. K. Dewdney's Wa-Tor described in Scientific American (1984). EcoMunch extends Wa-Tor by adding two more kinds of predators, and introduces terrain constraints on where predators and prey can live. The fast interplay of predator and prey populations make a visually interesting ecology.
Below is a screenshot of EcoMunch 2.1:
The simulation comes with more documentation. Even young children can have fun constructing terrains, adding some predators and prey, and seeing what happens.
EcoMunch - An Ecology "Chomp" Full Of Fun