The USPS says that yes you can send small batteries through the mail without any issues.
Under USPS postal regulations they do allow small batteries such as those batteries used to power cameras and flashlights are mailable domestically.
You just cannot send large batteries such as car batteries, motorcycle batteries etc through the USPS but small batteries such as those used in cameras, toys or batteries used in laptop computers or cell phones can be sent through the mail.
I've sent batteries through the mail for toys along with the toys when sending them to nephews and nieces and it's never been a problem.