You can make most nouns plural by simply adding an "s" at the end. For most rules in English grammar, however, there are lots of exceptions. Some plural nouns have a different spelling than their singular forms, and some don't change at all! Here's a worksheet that helps kids practice changing irregular nouns such as child, woman, and ox into their plural forms.
There are quite a few irregular nouns in English. Get more practice with Irregular Plural Nouns #2.