The normal school age habits are biting your nails, picking your nose, and twirling your hair. Then by adult hood we shift to our habits. Girls that bite their lip or chew gum. Men that stroke their beards, or bald men that rub their heads. Then there are those that have bizarre habits like squinting, clearing their throats, wrenching their necks, popping their knuckles.
Children can have weird ones too.
We shift habits, we don't stop them. The question is which one do you want. If you stomp out one, you will get another one you wish you never heard of. Leave your child alone for now and when they bite their nails, leave it alone and gently remind her not to suck her thumb. We have till 4-5 years old to get them to stop sucking their thumb.