Walking and eating are also universal. Because they can actually eat something in front of them and point to it. They can walk in front of the aliens and point

Walking and eating are also universal. Because they can actually eat something in front of them and point to it. They can walk in front of the aliens and point to the action and say "this is what we area talking about". Consuming nutrients and locomotion is not a bad first thing to focus on. Numbers are more abstract. They couldn't just draw a number "2" for example. Or show 2 apples because it might seem they are just talking about apples and not numbers.