As a starter I'd buy the book "Geography for Dummies" and you'll be amazed to find that they have moved DEL from Africa to Asia

Kidding aside you may find that siteseeing in India with small children may be quite rough on the children. The heat, mosquitoes, traffic, smog, crowds, etc., can take its toll on them. My children were the same age as your when we took them to DEL for their first visit and they swore that they'd never go back there again. They lied! They enjoyed the next trip when they were older and were able to appreciate the difference in culture between what they were used to in the US and India. We did a side trip to Agra that they enjoyed the second time around.