I was curious so I did a quick search and came up with this from the Colorado Dept of Labor & Employment: https://www.colorado.gov/pacific/cdle/breaks
Rest Periods
Every employer shall authorize and permit rest periods, which, insofar as practicable, shall be in the middle of each four-hour work period. A compensated ten-minute rest period for each four hours or major fractions thereof shall be permitted for all employees. Such rest periods shall not be deducted from the employee's wages. It is not necessary that the employee leave the premises for said rest period.
You could probably implement a site policy that would require employees to remain on premises during their break (and with the safety issues in working with children, it's probably a very good idea), but you would need to enforce that equally. If J can't leave to smoke, neither can Suzy to buy coffee. And if such a policy were implemented, you would want to hold a staff meeting informing everyone and letting them know that a violation could result in disciplinary action. Since you are new, it would probably be wise to run this by your manager first.
Of course, that would not necessarily solve the smoke smell issue because J could just smoke up before clocking in or during lunch when he can leave the premises. That issue I think would best be addressed with the help of your manager and HR to make sure you cover all your bases and tackle it in a legal manner.
(It's worth noting that many employers give a 15-minute break even though the legal requirement is only 10 minutes. Every company I've ever worked for has done that. So you might want to ask your manager what the official or unofficial policy is in your district.)