I would say no, not technically. They need a certain amount of calories, fat (proportionately more than adults) and calcium in their diets, and milk is probably a pretty easy way to squeeze that in on a daily basis. But with a bit more effort and planning, you can definitely get all of that from other sources. I think it just tends to be a bit harder, but definitely possible, as there are plenty of kids who are allergic or intolerant or vegan or whatever who don't drink milk. So if your LO doesn't like it, I don't think it's worth forcing it, and there are definitely other options - oily fish, leafy greens, legumes or cheese/yogurt if you do dairy, plus just being mindful to add butter, olive oil, coconut oil or whatever to cooking to add in a bit of extra healthy fats.