I bit my nails until I was well into adulthood and still do unless they have been manicured professionally. It's a habit that is very hard to break. Painting, all the goop that tastes bad, etc., none of that works. The best thing to do at this age is to keep them trimmed and make sure that any rough edges are filed. She's not really old enough for a professional manicure, but doing one at home once a week would be a good way to encourage her to stop. Rough edges and dry skin around the nail are what keep biters biting. When mine are longer, I won't mess with them, but if one gets broken, it becomes a problem.