I do food chains and food webs with my children (when we study our salt marsh). The children wear a card with a picture and a name of an animal/fish/.. on it. Each card has the number of strings attached to the bottom of it as children who are animals that they eat. (An animal high on the food chain might have three strings attached and the animal at the bottom of the food chain would not have any strings attached). They can use the strings to connect together making a straight food chain, and more complex food webs.
I have posters with an example of a food chain and a food web to show them during the introduction.