Talking to kids about weight can be a sensitive topic, no matter their age, relationship, comfort level, etc. But, if you have enrolled them in the ProActive Kids program, you should sit down and talk with them about the program and its goals – creating a healthier lifestyle for everyone in the family.
We know you don’t want to say or do the wrong thing and risk alienating or hurting them.
We will go through many communication lessons onsite during the program and this will be your first PAK lesson. Sit down and talk with your child about getting healthy and learning how to live a healthy lifestyle! Let them know you care about them and are doing this for them and their health. Remind them that other kids will be there to go through this with them and they will have a lot of fun, while learning a lot.
If they are hesitant, ask them to just give it a try for the first week. The kids have so much fun in the first day or two that they forget their worries quickly. Getting them there on the first day is the hardest part! 
Here is a great article on how to talk to your child about their weight. It has a lot of good lessons for parents to help you embark on this potentially difficult conversation. http://www.webmd.com/parenting/raising-fit-kids/mood/talking-kids-about-weight?page=1