There is growing evidence that the prevalence of overweight/obesity is increasing in both developed and developing countries. Furthermore, complications of overweight/obesity occurring during childhood may influence the healthy status in adulthood. On these grounds, here we have enrolled 15 overweight/obese, 15 normal weight and 15 underweight children. A series of parameters, such as eating habits thanks to the Body Mass Index (BMI), Youth Healthy Eating Index (YHEI), activity and inactivity, salivary nitric oxide concentration (NO), were determined and correlated between them. In particular, correlations of BMI with YHEI, activity/inactivity, and correlations of salivary NO concentration with YHEI, BMI and activity/inactivity were evaluated. In overweight and obese subjects, a positive correlation between BMI, YHEI, inactivity and salivary NO concentrations was shown, while an inverse correlation was documented between BMI and activity in overweight/obese group. In normal weight and in underweight children, respectively, a positive correlation between lower BMI, YHEI, salivary NO concentration, and higher physical activity was observed.