Dr. Greene's Answer
Allergies to the proteins in cow’s milk are not uncommon at 18 months old. Thankfully, most kids that age will outgrow them. In the meantime, breast milk is, of course, the best milk. Beyond this, kids don’t necessarily need milk at all: though they do need good sources of protein, calcium, vitamin D, and fat (as well as Vitamin A, Vitamin B12, Pantothenic acid, and Ribloflavin – also found in cow’s milk). Cow’s milk just happens to be a very convenient way to get them all.
Of the other milk’s out there, such as soy milk, rice milk, oat milk, almond milk, or other mammal milks, such as goat milk, they all can be found with plenty of calcium and vitamin D and fat – 3 of the biggies. So the biggest consideration is usually protein. Almond milk and rice milk are often very low in protein (1 gram per 8 ounces – unlike milk which is 1 gram per ounce or 8 grams per 8 ounces).
If your toddler’s getting plenty of protein, or protein-fortified milk, then those milks are fine. At 18 months you want kids to get at least 13 grams of protein per day. That’s easy with soy milk and not hard with oat milk. Other mammal milks are good bundles of nutrients, but of those commonly available in the US, if a child is allergic to one she is usually allergic to the other (mare’s milk, camel’s milk, and reindeer milk could be great choices, if available).
I like soy milk from whole soy beans (and I think choosing whole soy beans, in moderation – say 16 ounces a day – is the answer to the soy controversy. I hope that gets you started!