Applying something on the nails, hardly ever works, especially for older kids. They go to the bathroom and wash it off. Usually there is a reason behind it and yes, it is definitely unattractive!
I bit my fingernails and actually taught myself to stop it! It was my own willingness and desire to stop. It worked! I never ever did it again.
I believe therefore it is a mental readiness and willingness to overcome it, that makes it possible. I also don't believe others telling you how they dislike it matters less, it is about yourself - and I am sure your kids don't like the look of it either. So approach them by making them self aware of the appearance of it. They change if they feel there is a need and have the courage and power to do so. It has to come from their mind and point of view.
Some do the biting unconsciously, and never are fully aware of the appearance, but by making them think about it, they might become aware.
I am personally disgusted by the suggestion of putting fingernails on a plate! It is offensive and won't receive a good reaction from your kid, plus eventually a damaged relationship. They might feel that you are making fun of them or something like that. I find it totally inappropriate!