Studies of the relationship of copper (Cu), magnesium (Mg), and calcium (Ca) levels with blood pressure in children are limited. This cross-sectional study included 443 children aged 6–9 years from Guangzhou, China. Plasma concentrations of Cu, Mg, and Ca were measured by inductively coupled plasma mass spectrometry. Systolic blood pressure (SBP) and diastolic blood pressure (DBP) were measured with an electronic sphygmomanometer. Elevated SBP and DBP were defined as the 90th percentile or greater (age and sex specific) of reference values for Chinese children. Abnormal blood pressure (ABP) was defined as an elevated SBP and/or DBP. The plasma Cu concentration was positively correlated with blood pressure, body mass index (BMI), and the C-reactive protein (CRP) level, whereas the plasma Mg and Ca concentrations were negatively correlated with SBP. A higher Cu concentration (T3 vs. T1) was associated with a higher risk of ABP (odds ratio (OR) 2.55, 95% confidence interval (CI) 1.20 to 5.40). In contrast, children in the top tertiles of Mg (OR 0.42, 95% CI 0.18 to 0.98) and Ca (OR 0.28, 95% CI 0.11 to 0.70) concentrations showed lower risks of ABP than those in the bottom tertiles. Path analysis showed that the CRP levels and BMI mediated the associations between the Cu concentration and ABP. We found that higher plasma Mg and Ca concentrations were negatively related to blood pressure in children aged 6–9 years. In contrast, a higher plasma Cu concentration was positively correlated with the risk of ABP, and the association was mediated by CRP and BMI.