"Bred these dogs in China" is not a complete sentence, but rather a sentence fragment. Every sentence must include both a subject and a predicate (which contains a verb), and in this example there is only a predicate. Think of the subject as the person or thing performing the action, and the predicate as the action being performed.
To make the sentence complete, add a subject in front of the verb. Asking yourself who or what bred the dogs will help you figure out what the subject should be.
Here are a few examples of ways to complete the sentence:
I bred these dogs in China. (I is the subject)
Thomas bred these dogs in China. (Thomas is the subject)
The company bred these dogs in China. (The company is the subject)
For further practice with sentence grammar, check out the worksheets I've linked to below. The first link goes to a selection of worksheets on subjects and predicates, the second helps with identifying sentence fragments, the third is all about verbs, and the last one helps kids identify the subject of a sentence.