A lot of children have a bad habit of picking their nose. Even though it's a common behaviour for children it's probably one of the most annoying behaviours to parents. Our four year old picks her nose sometimes and when my husband and I see her do it we tell her to stop. We always tell her to wash her hands immediately after and use a tissue if necessary. Recently our 20 month old started copying that behaviour and now picks her nose too.
Children usually pick their nose because they have mucus stuck in there from an infection or they're just bored. Mucus contains a lot of germs and picking their nose can cause nosebleeds. My husband and I are trying to nip it in the bud. The most we can do is encourage our children to use a tissue and wait to see when they grow out of this bad habit.