Yes, you must get the Visa allotted before visiting India. All visitors from foreign countries except the visitors from Bhutan, Maldives and Nepal, need to have a Visa to enter India.
There is a list of over 200 countries made by Indian consulate that are eligible for applying for the e-Visa for visiting India.
The visitors should apply for the India visa 3 - 4 weeks before your journey date. Although most of the times the Visa allotment process gets completed within a week, it is better to have it issued well before time.
Yes, they also need to get Visa allotted for them and undersigned by their parents, in case, they are minor.
A visitor can get two time entry as double entry is permitted on e-Tourist Visa issued by India.
Visitors are allowed to stay on e-visa for 60 days, with the double entry on e-Tourist Visa issued by India.