What you say might depend on their age and understanding. If you’re telling multiple children with significant age gaps, it might be helpful to tell them separately. This way you can adjust what you say to their level of understanding.
For many children, you can explain pulmonary fibrosis by saying that your lungs are not working as well as most people’s, making breathing harder for you. You might want to describe how this affects your daily life so they understand that you might need to do some things differently.