An alternative way to milk cows on touchscreen versions is to press and hold on the screen with your finger on the cow without moving your finger across the screen. However, if the button to milk the cow isn’t showing up, I would consider creating a bug report. I’m going to test this for myself right now. Edit, I also couldn’t get the button to show up. I’ve included a picture of this button, which does still show up in the most recent version of Minecraft. I’m going to make a bug report about this issue.