The two are caused by different pathogens. Mycoplasma pneumoniae pneumonia is caused by an infection with Mycoplasma pneumoniae, whereas the pathogen of Chlamydia pneumoniae pneumonia is Chlamydia pneumoniae.
Symptoms of Chlamydia pneumonia are much milder, mainly manifesting as fever and chills, dry cough, sore throat and malaise, whereas mycoplasma pneumonia can have a persistent paroxysmal cough, cough with sputum and the child may have more severe and even life-threatening symptoms.