Most children can sort nouns, verbs and adjectives when given a list of words (go here for this type of worksheet). They can even identify nouns in a sentence or verbs in another sentence or adjectives separately in a different sentence. But somehow they have difficulty identifying them in the context of a single sentence.
I have searched for the type of worksheets that ask students to circle the noun, underline the verb and cross out the adjectives in a single sentence. There aren't that many online so I've made my own.
I've taken some of the text from my children's kindle ebook 'All About Cats for Little Kids' and made a few adjustments where necessary. You can take a look at the book here. (You don't need a kindle to read it. You can download the Kindle app for free onto your computer or ipad/android.)
Download the worksheet (pdf)