Need help with my slightly overweight child focus on diet or exercise first?

Both: No one loses weight thru exercise, only thru diet. However, exercise curbs hunger, tones muscles, maintains metabolism which tends to slow with reduced caloric intake. Moreover it puts one in the mindset of being healthy, weight conscious, and motivated to follow a healthy diet. The 2 go together like peas and carrots. Don't overdo it with a "slightly overweight child". Moderation! moderation!

Both: Need to focus on both avenues. Limit screen time and encourage outdoor activities even if it is just walking. Eliminate sodas and dairy. Encourage increase water consumption. Remove sweets and snacks and replace with fruits and vegetables.