Although I've never been to nor lived in Delhi, I can tell you that Delhi indeed does have one of the worst cases of air pollution in the world. In fact, there are just two other cities that are worse. The air is filled with something called "particulate matter", comprising of smoke, dust, dirt and chemicals.
Killing some 1.5 million people a year in the country, it is a huge problem not only in Delhi but across the whole of India. It will be particularly risky taking your child. According to the World Health Organisation, some 2.2 million children suffer irreversible damage to their lungs. Those with chronic respatory illnesses such as asthma are particularly vulnerable to harm. For both adults and children alike, doctors say it is the equivalent of smoking 10 cigarettes a day.
At the end of 2016, the problem became so bad that schools in Delhi were closed for three days and factories actively shut down in an incident known as The Great Smog of Delhi. There is also evidence that has been unearthed by the Agency for Research on Cancer to suggest that it contributes to both lung and bladder cancer too.
Hope this has given you some insight.