S.K.
G.,
I have a very active almost 7 year old who started having body odor this fall after football practice. I changed his shampoo and body wash from "kids" kind to adult kind had had him shower every night. I also monitored his hair washing and we went over the importance of being thorough when showering. He was getting sloppy about washing ALL of his hair and EVERY part of his body. That solved the problem. I wouldn't have him use deoderant yet.
My daughter is 8 and started having the same issues with her breath. We switched from kids' toothpaste to adult toothpaste and she started using a Listerine product for kids 1-2 times per day. That helped.
Good luck,
S.