Milk alternatives don't have any benefits in children. Milk drinking in children is thought to induce growth. You can't overdo it though as it interferes with iron absorption.
If we go purely on factors contributing to obesity, plant milks may not necessarily be better. Many plant milks are carb heavy with not much in the way of fat or protein, so it won’t make you feel as full as milk.
I'm not a milk or milk alternative drinker but when I've looked at things like Oatly they seem to have at least as many calories and sugar as something like skim milk.
At least the oat milk brand I like to drink (Alpro) has exactly the same amount of calories as normal cow milk, which makes kinda sense, as they use additional oil to get to the identical fat content :D
So replacing it will not change anything in that regard, we just must be aware that (any) milk is (liquid) food and not a drink. Same problem as with softdrinks or fruit juices.
Anecdata: I regularly drank (cow) milk as a child and now I'm over 2 meters in height. But of course correlation with n=1 is not causation, and I'm sure many people here are equally large without a drop of milk. :D
Milk should be the beverage of choice for growing kids who do not have contraindications. 3.25% milk is recommended up until 2 years of age, and to continue into older age if there are no issues with obesity.
I have my doubts about such recommendations; aside from the influence of the dairy lobby, how many such nutrition guidelines account for high-quality non-dairy alternative sources of protein and calcium?
The only dairy alternative that has comparable protein quality (DIAAS) is soy, and AFAIK there aren't many studies of soy milk given to toddlers/small kids.
Consumption of a mostly soy diet by small children is undesirable because of the presence of phytoestrogens in soy.
Pulses would come close second as a protein source, but a child would have to consistently eat larger servings in order to keep up with other meat-eating-and-milk-drinking children. Large portions are not always realistic in ensuring adequate protein intake in kids, who will occasionally go through phases of fussy eating.
Adopting vegetarian or vegan habits is great when periods of significant growth stop (and maybe this is what humans are meant to do, rather than to keep eating animal products in adulthood).
However, from my own experience, meat and milk is a necessity for avoiding malnourishment in little humans undergoing development.
I drink a ton of milk, maybe a third of a gallon a day. Always thought the obsession with counting calories was a little misplaced. It always seems to be very fit people drinking a ton of milk. People think that junk like Special K is healthy and great stuff like butter, eggs, and milk is bad for you. Tell that to French people who live to 120.
As I understand, the reason why children are encouraged to drink milk is mostly about calcium. Many milk alternatives now have the same or more calcium per litre as cow's milk. When you say "growth" is this about bone (calcium) or muscle (protein) growth? In my experience, protein per litre is generally lower for milk alternatives.