'Stop and grow' does still exist and it can be bought at Boots. Have a look at Nailbiting by Googling it. It is the normal habit of most children up until around 14/15. You are lucky that you had a brace.
However, for some it continues into Adulthood and there is talk of reclassifying it as self harming. I think it was in my case.
I have been a naibiter all my life. I am an anxious person and it comforted and soothed me. It looks so disgusting and I was ashamed of doing it but it seemed impossible to stop. I wondered if I had some sort of vitamin deficiency. However, my Father was toxic and although netither he or my Mother bit their nails, my Mother was very anxious and picked her skin.
I actually stopped around 7 years ago as I had acrylic nails applied constantly and they are very difficult to bite off. I was extremely determined to stop and had regular manicures etc.
It is the first thing I notice about anyone. Princess Diana bit her nails initially. I was interested to see her engagement photos as she curled her fingers towards her palms to hide her bitten nails. Later she must have had weekly manicures as she stopped.
You do see very old people with bitten nails. Where I worked, out of around 42 people four of us, who were adults had severely bitten nails.
Like you I dreaded any of my children biting and when one showed signs of starting( they were two) I crept into their bedroom when they were asleep and applied 'Stop and Grow'. They don't bite their nails now but they pick.
I have accepted that I am anxious. It is inherited.
I wish that I could be more helpful. Perhaps I could have been put on some sort of therapy when I was a child as being anxious makes it difficult to learn. I went on a Mindful course and that was calming.
Perhaps a HV might have some better advice for you. X Sorry