I tried not to let dd1's nail biting get to me (trying to just understand it as an anxious habit) - I did get her some of her own nail trimmers and hand lotion at the time when it seemed to be getting really intense, and would suggest she trim her nails or use the lotion instead if I saw her biting her nails. She actually totally stopped for a time. Still bites her nails sometimes now, but she's also able to take care of them well too and it has cut down on the biting overall.
I tried to point out how sometimes, biting can leave your nails prone to bleeding or other spots that can hurt or get hurt easily, and it would be good to use the trimmers and lotion to help avoid that. (She was 4 when I got her the nail trimmers, and actually ended up managing them fine without hurting herself, in case you're worried about that - if you don't think your kiddo is ready for trimmers, a good nail file might be helpful instead).
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