Sometimes when kids chew with their mouth open, pick their nose, or bite their nails, parents let these behaviours slide. And while it is true that some bad habits will disappear over time, you may need to partner with your child to break the habit, especially if they are experiencing some adverse effects from it.
For instance, nail-biting may lead to infected nails, or nose picking may cause nosebleeds (or teasing from peers). To help your child break their bad habits, you work with them on making a change.
Ask them how they want to approach changing their behaviour and offer suggestions for alternate behaviours. Offer your support and encouragement and provide rewards if they are motivating for your child.