While in the child's diary, click Food to expand.

If you haven't created a menu, you can pick items from your food list, by clicking on + next to each mealtime.

Select the new food item from your list and click Add.

You can add more than one item, by holding down ctrl or cmd while selecting.

Once you have populated the food section, click Save. 

How do I add items to the food list?
​How do I create / use menu templates?