I live in the UK. The sun makes approximately six appearances per year, usually in a 10-day period in August when everyone is on holiday. Vitamin D is something I think about a lot as a parent in a northern climate because the research on its importance is genuinely compelling. Vitamin D is essential for calcium absorption and bone health, immune function, and muscle development. Toddlers who are deficient in vitamin D are at risk of rickets, a condition that affects bone development. It is more common than most people realise in northern latitudes and in children with darker skin tones who need more sun exposure to produce the same amount of vitamin D. In the UK, the NHS recommends that all children aged 1 to 4 take a daily supplement containing 10 micrograms of vitamin D. This is regardless of diet or sun exposure. In the US, the AAP recommends 600 IU per day for children over 1. The main sources of vitamin D are sunlight on skin, oily fish including salmon and mackerel, eggs, and fortified foods like some cereals and plant milks. In most northern countries, sunlight alone is not sufficient for vitamin D synthesis during the winter months regardless of how much time children spend outside. A daily vitamin D supplement is genuinely one of the easiest and most evidence-based things you can do for your toddler is health. Look for a combined vitamin A, C, and D supplement designed for children, or a standalone vitamin D drop. They are inexpensive and widely available.