Vitamin D is vital for keeping your bones, teeth and muscles healthy. A lack of Vitamin D can cause rickets, especially in children. The people most at risk of Vitamin D deficiency are children under the age of four and babies. https://www.rnoh.nhs.uk/services/children-and-adolescents/vitamin-d-children
Usually, in the summer months, you should get enough Vitamin D from sunlight; however, if you or your child are inside the majority of the time, then you should consider taking daily vitamin tablets in order to support normal bone health. You can also get Vitamin D from foods such as oily fish, red meat, liver and egg yolks. https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/