Yikes!  This does sound stressful.  However, this also sounds like pretty common behavior for 3rd graders after school.  It is hard for a 3rd grader to have to sit down all day and focus on learning and school work.  After school time is typically a time when kids need to unwind and destress after a long day of school.  Please don't beat yourself up too much or be too hard on the kids for this behavior.  After school time is typically unstructured and chaotic time and changing that behavior is very difficult.  

The best way you can get them to pay attention is to try and get them involved in an engaging way.  Kids repond well when they have activities and goals to engage in fully with all of their senses.  If you can find exercises and activities that engage their entire body, allow them to be a little loud and rambunctious, and keep their minds working, this will be the best way to set up success.  

If you continue to struggle, than it may be good to try and speak with a supervisor or an administrative staff member about how best to approach the group.  It may also be good to speak with their regular teachers and see if there are techniques that they use which you could utilize to create more control and order.  Also if you could somehow reduce the number of students in the program, than you may have more luck gaining control.

We wish you luck and patience in this difficult endeavor!

Counselor, Dominic
Boys Town National Hotline-A resource for parents and teens
1-800-448-3000
www.parenting.org