It depends on their diet! For most young children, their food intake tends to balance out over time. Each day will likely not contain appropriate amounts of grains, protein, dairy, fruit and vegetables. But over the span of a week, if your child is offered these healthy foods on a daily basis, their food intake will likely meet their needs.
As for multivitamins, if your child is a pickier eater, then they can’t hurt. They provide a bit of nutritional “insurance” although they don’t make up for an unbalanced diet. Children’s multivitamins contain small doses of a variety of vitamins and minerals, so they are not harmful, and may be beneficial for your child. But are also not really necessary for most kids. If it makes you feel better to know your child is taking a multivitamin, go ahead!
If your family does not eat fish twice a week, I often recommend a fish oil supplement. Not just “omega 3,” as this may contain more ALA (from flax, canola, etc.), rather than the DHA omega 3 type of fat that is most important for development.
Also, if it is winter in Canada, I would definitely recommend the whole family take a vitamin D supplement. From October through March, our body cannot make enough vitamin D from skin contact with the sun, due to our latitude. If you are especially vigilant about sunscreen in the summer, this also blocks vitamin D absorption through the skin, so you can continue the supplement. While dosage amount recommendations are conflicting among health organizations, 400IU per day should be fine for a child. I prefer “D Drops” or products with little additives that require only a one-drop dose.