@BabyInTheJungle
I buy the M&S chocolate oat milk for my 5yo DS as a treat. Am I doing him more harm than good? In my head I somehow thought it was vaguely healthy...
Oat milk is likely fortified with calcium, vitamin d so not completely 'unhealthy'. Full fat milk is going to have more calories so if that is a concern, it's healthier. (Although five year olds should have a reasonable calorie intake and a lot of us struggle getting enough into them.)
The only milk alternative I know of that meets most of the nutritional needs that cow's milk does is alpro growing up milk. It's fortified especially for children's mineral needs. However, it doesn't have omega 3, but as long as your child has other sources of this or a multivitamin, it's not a problem.
The thing about cows milk is it's a full meal for a rapidly growing calf, so it's going to have all the nutrients in it naturally like breast milk does. Not drinking cows milk and using oat milk etc is fine as long as you make sure you get the nutrients in the rest of the diet. And not all milks are equally nutritional value so it's worth shopping around to find something that covers most bases. (Omega 3 never seems to be used probably because it spoils food quickly).
As for chocolate milk, I occasionally give my toddler the strawberry soya ones. No they are not as good as the growing up milk, but they do nicely as a treat.