the dirty thing that comes out from the eyes if somebody just woke up what do you call it in English?
With a child you would say "sand".
Although NZ English does have "the sandman", in my 41 years I have NEVER heard that substance called "sand". In NZE it is always "sleep", as in, "you've still got some sleep in your eyes". That's what it's called even among children, and speaking of sand would make listeners think of the beach. Vive la différence!