Pneumonia: Given the symptoms of decreased air entry to the lower lobes bilaterally, fever, and low oxygen saturation (SpO2 of 90-91% on room air), pneumonia is the most likely diagnosis. The clinical presentation is consistent with a lower respiratory tract infection, which is common in children and can be caused by viral or bacterial pathogens.

Bronchiolitis: Although more common in younger children, bronchiolitis can occur in older children, especially if they have underlying respiratory conditions. It presents with similar symptoms, including decreased air entry and hypoxia.
Asthma Exacerbation: If the child has a history of asthma, an exacerbation could present with decreased air entry, especially if there's an element of airway obstruction. However, the presence of fever might lean more towards an infectious cause.
Pleural Effusion: This could be a complication of pneumonia or another condition, leading to decreased air entry due to fluid accumulation in the pleural space.

Pneumothorax: A life-threatening condition that requires immediate intervention. Although less likely, it could present with decreased air entry and hypoxia, especially if the child is very unwell.
Sepsis: Given the child's fever and appearance of being very unwell, sepsis should be considered, particularly if there's a suspicion of bacterial pneumonia or another severe infection.
Foreign Body Aspiration: Although more typical in younger children, it's a critical diagnosis not to miss, as it can cause acute respiratory distress and decreased air entry, especially if the object is lodged in a mainstem bronchus.

Cystic Fibrosis Exacerbation: If the child has cystic fibrosis, an exacerbation could present with decreased air entry and hypoxia, among other symptoms.
Interstitial Lung Disease: A broad category of diseases affecting the lung interstitium, which could present with chronic or acute symptoms, including decreased air entry and hypoxia, though this would be less common in an acute setting.
Congenital Heart Disease with Pulmonary Complications: Certain congenital heart diseases can lead to pulmonary complications, including decreased air entry and hypoxia, especially if there's significant pulmonary hypertension or other complications.