Nearly all children who have a chest x-ray performed will have a degree of peribronchial thickening in the perihilar regions. It's a function of the fact that only sick children get a chest x-ray.

Features here are of a viral infection and with the history and age, the most likely diagnosis is bronchiolitis.

Bronchiolitis is very common and a chest x-ray is useful to determine whether there is any additional consolidation of collapse that might require further treatment, e.g. for superimposed bacterial chest infection.

The large anterior mediastinal mass is normal - it's the thymus with its wavy edges (wave sign of Mulvey).