Unfortunately, there is nothing you can do to ensure this won't happen. It is not uncommon for people with BFRBs to pick/pull from other areas of their body, so don't be alarmed if your child starts to do so. It's very hard to stop pulling/picking from an area once you start, so don't expect your child to automatically stop. The best thing you can do is offer your support and unconditional love.
Do not scold your child or punish them for their BFRBs. Your child's BFRB is not something they can control, and it is not their fault. It's not your fault either. BFRBs are not self-harm. BFRBs are not caused by trauma.
It is critically important that you keep the lines of communication open with your child in order to support their overall physical health and emotional well-being. The more informed you become about BFRBs, the better advocate you can be for your child.