Vitamins are important for your body to work well. If your child is eating a healthy diet with a variety of foods based on Canada's Food Guide, they likely won't need a supplement (aside from vitamin D through the winter). 

There are some common findings of low vitamins and minerals in children with specific mental health concerns (e.g. because of medications or picky eating). It's important for a doctor or dietitian to monitor if there are concerns of major food deficiencies. 

• Frequently, children are low in iron (autism spectrum disorder, ADHD) - talk to your doctor about checking, especially if your child does not eat meat products. 
• Often, children are not getting enough calcium in their diet. This is especially true if dairy has been cut from the diet. See here for alternatives to dairy to keep bones strong, if your child does not take dairy products. 

At the moment the evidence is not strong regarding the benefits of nutritional supplements, but if you decide to try them consider: 

• Giving your child a daily multivitamin if your child is not eating a balanced diet (e.g. missing an entire food group like fruit or veggies). 
• If your child is taking multiple supplements, check to make sure they aren't getting more than the recommended upper limits (UIs). 
• Reminder: sometimes supplements can interact with medications, check with your physician or pharmacist.