You have to make a rough estimate. For example if you are offering spaghetti or pasta in your menu, you have to note that a kilo of pasta is good for eight to ten servings for adults or eighty grams of plain pasta if accompanied with two half slices of garlic bread on the side per serving, one hundred grams of plain pasta if without bread per serving for adults.
Children consumes less food so a kilo of pasta will yield fifteen servings or 65 grams of plain pasta per serving with or without the bread. You weigh first the plain pasta for each serving before you put on the sauce.
For toddlers estimate at 50 grams per serving and for bigger ones at 75 grams per serving.