I completed a C/A fellowship and I'm 4 weeks away from finishing a forensic fellowship.
I have a lot of mixed feelings about doing child forensic cases. There are a lot of areas you can focus on, but the constant work is typically in either doing juvenile delinquent work or parental rights/custody cases. Personally, custody cases are not my cup of tea. They can be very contentious and nasty. However, because there are such few child psychiatrist/forensic trained psychiatrists, you can make a lot of money once you developed professional connections, as whooper said. Personally, I don't think it's worth the money and, at this point, I do not plan on picking up that kind of work.
One thing to keep in mind is that doing a forensic fellowship does not mean you're going to step out of the fellowship and just be a forensic psychiatrist, unless you're doing treatment. Also, keep in mind that when you're being asked to do an evaluation or a forensic case, the main skills and expertise you're bringing is as a clinical psychiatrist. The forensic training helps you know how to ask the right questions, the legal issues involved in the case, and, probably by far the most important reason,
is how to write a good report. I know people who have done child fellowships with no forensic background and pick up forensic cases because the attorneys just need someone with the degree and child background. Reading their reports makes me cringe. They also tend to get involved in stuff that they're really not prepared for because they don't have the background to know how to narrow the evaluation/report to specific questions that are useful for the court. The representing attorney doesn't mind, because often they're fishing for anything to help their case and the more expansive you are in your report, the more they can try to pull stuff out that may or may not be what you wanted to convey to the court. It can get tricky and if you get on the stand, the cross exam can get nasty.