Ok, let me put on my teacher's hat... Done. Now, on with that explanation: I'll start by saying that the rules dealing with punctuation are not as many or as strict as those that deal with other linguistic domains, and they are often tricky when it comes to writing and reporting dialogue. As you know, my dear April, a comma must be used between the dialogue and the words used to identify the speaker (this is called tag line):
"Michele is an amazing editor", said Rosa.
Now, when the tag line interrupts a sentence, it should be set off by commas:
"April does not like it", Rosa said,"when she isn't right about something".
Note that the first letter of the second half of the sentence is in lower case.
When one single speaker says two different things/sentences and you decide to place the tag line between them, again two commas are used, as in your example:
"That’s so sweet", Brice mocked, "I think I just got a cavity".
You could use a period after the word 'mocked' if you chose to turn the second sentence Brice said into a completely separate statement, with its own tag line, as in
"That’s so sweet", Brice mocked. "I think I just got a cavity", she added.
I must confess that your question got me thinking for a while, but the only other instance in which I believe you can use a period there is if the second sentence belonged to a different semantic category: sentences can be classified according to meaning, thus you have declarative (sentences that make statements), imperative (a sentence that gives a command or gives a request), interrogative (sentences that ask questions) and exclamatory (a complete sentence that expresses surprise, excitement or some other strong emotion). In your example Brice says two declarative sentences, but if she had said first a statement and then some other type of sentence, like a question, a period could probably be used after the tag line:
"That's so sweet", Brice mocked. "Is this a cavity I just got?"
I am not really sure about this though. It just makes some sort of sense to me. In conclusion, Michele was right, so, April, get over it and congratulate yourself on having an amazing editor
