Passe o mouse sobre a criança do pai Mui React

<Box
  sx={{
    width: 300,
    height: 300,
    backgroundColor: "darkblue",
    ":hover .child": {
      backgroundColor: "orange"
    }
  }}
>
  <Box className="child" sx={{ width: 200, height: 200 }} />
</Box>

https://bilalahmed-dev.azeemlab.com/
bilalahmed_dev