My daughter bit her nails too, but of course it's easier for kids to break a habit than it is for adult that have been doing something for many years. The main problem is that many times we do something like this, we aren't even awake we are doing it.
There are a lot of gimmicks out there that have fancy formulas, but really there is a very simple and inexpensive product called THUM that has cayenne in it. You can get it at a drugstore or even walmart (you may have to ask for it at the pharmacy.) It is safe for kids and adults and helps both! You bush it on just like nail polish.....just make sure you never get it in your eyes, because even after it has dried on your nails, it can sting your eyes.
Remember it takes an average of a month to break a habit, so give yourself some time. As long as you remember to keep putting on the THUMs product, I'm sure you will remember that you are biting your nails when your mouth starts to burn