What about kids who are overweight or unfit?

One of the hallmarks of CrossFit Kids & Teens is inclusion. Training in a group of peers can have a profound positive effect, increasing daily compliance with the program, pushing mental and physical limits, and maintaining a positive sense of self (Beets et al. and Vorhees et al.). Children that are deconditioned will also benefit from exercising in a group. The key to incorporating children with various physical abilities is in modulating the planned workout, often referred to as scaling, to accommodate differing abilities and capacities. The immediate goal is inclusion and maintenance of the general movement patterns, the long-term goal is to have each individual continue to improve and require a decreasing amount of scaling.
Beets MW et al. Social support and youth physical activity: The role of provider and type.American Journal of Health Behavior 30(3): 278-289, May 2006. http://www.ncbi.nlm.nih.gov/pubmed/16712442

Vorhees CC et al. The role of peer social network factors and physical activity in adolescent girls. American Journal of Health Behavior 29(2): 183-190, 2005. http://www.ncbi.nlm.nih.gov/pubmed/15698985