Roisin,
this strikes me as a bit high-level for a junior class. Food chains appear as a curricular item for 3rd/4th class in Living Things/Plant and Animal Life and again in 5th and 6th class.
With support a younger class might look at seeds and what they need to start growing, or look at plant and how it responds to light (both curricular items for 1st/2nd class), but I think a full ecosystem would be better left to second level.