There is no specialty of "child abuse nursing," but pediatric nurses deal with injuries related to abuse a lot, and child psychiatric nurses see a lot of abused kids. As you note, there are also roles for nurses in forensics, although I've never heard of anyone specializing in
child forensics (doesn't mean it doesn't exist -- just that
I've haven't heard of it so far

).
I know some child psych nurses take the SANE (sexual assualt nurse examiner) training and specialize in doing the physical exams and evidence collection on children that have been raped/assaulted.
There are many, many possibilities for specialization and different roles "out there" (and the possibilities are changing and increasing all the time!), and I'm sure you'll pick up a lot of information as you got through school. If you want to work with abused children, I
guarantee you you'll have plenty of opportunities.
Best wishes for your studies and new adventure!