In theory, this proposal seems like a great protection for children. I don't see it being enforced very well.

Sponsored by Indiana senators from both sides of the aisle, the bill would make it illegal to smoke inside a car with children, 6 and under, inside. According to wifr.com,

If passed, Indiana Senate Bill 34​ would make smoking in a car with a child under 6 years old a crime. The bill would make a first-time offense a Class B infraction, which is punishable by up to a $1,000 fine.

If you get caught smoking in the car with kids three times over the course of a year, you could be looking at a $10,000 fine.