Scenario 1: The setting is a pond in the woods that is drying out in a very hot year.
Characters (adapt this to your local habitat as appropriate):
Frogs (who eat mosquitoes and need water for their eggs and tadpoles to survive)
Female Mosquitoes (who need blood to lay eggs & whose larvae need water to survive)
Water Snakes (who eat frogs and need water to survive)
Human Children (who witness the drying out pond and need to decide how they will respond in the situation)
Scenario 2: A stray cat with kittens needs a home.
Parents (who aren't sure they want a cat)
Cat (stray that needs a home)
Kittens (strays that need homes)
Mouse (who already lives in the house with her babies)
Dog (who already lives in the house)
Scenario 3: Baby bird falls out of its nest.
Baby Bird (has fallen out of nest)
Baby Bird's Brothers and Sisters (still in nest)
Large Predator bird (hungry)
Worms (or other insect the birds might eat)