It's not the act of *drinking milk* that is unethical - it's the act of consuming a product that was not produced in an ethical manner.

Vegan mothers don't take milk from lactating cows just like they don't forcibly take milk from **other lactating humans**.

Vegans abstain from consuming animal products because the production of those animal products involves suffering on the part of the animals that produced them. The whole idea of veganism is to minimise (as much as is reasonable) the suffering caused by our lifestyle choices.

I grew up on a goat dairy, which is almost operationally identical to a cow dairy. Taking milk from cows involves (amongst other things):

* Dehorning the cow. A [red hot iron](https://i.sstatic.net/0RZaN.jpg) (much like the old car cigarette lighters, but larger) is pushed into the skull where the horns usually grow.
* Removing the calf from the mother. This involves [emotional stress](https://www.wired.com/2014/06/the-emotional-lives-of-dairy-cows/) on the part of the calf and the mother which may last weeks, very much like human grieving (we grieve with the same mammalian lower-brain structures).
* Killing the calf if it is a male (and sometimes if it is a female). Sometimes they are killed immediately, other times they're grown a bit.
* Growing the females to supplement the diary herd.
* Artificially selecting cows which produce abnormally large amounts of milk. The massive amounts of milk that they produce is unhealthy and renders the cow's body [susceptible to disease](http://www.animalsaustralia.org/factsheets/dairy_cows.php) (and thus antibiotics are essential to keeping them alive).

So now that you understand the reason vegans don't drink milk from farmed animals, I hope it's obvious why vegans are okay with breastfeeding their children: It doesn't cause other animals to suffer.