If you need to describe a sandpit, I would suggest that you first start by stating where the sandpit is. Is it in someone's garden? Or is it in a playground? Is it surrounded by grass, or is it built on a concrete floor? You could also describe whether it is situated in the shade or in the sun.
As there are different shapes of sandpits, you could then describe the shape. It could be round or rectangular, or perhaps it is shaped like an octagon. Following on from this, I would focus on the material it is made from and its color. You could describe whether the sandpit is made from wood, or perhaps from metal or plastic. Some sandpits are decorated in bright colours, and others simply have the color of the material they are made from. Therefore, it might be also a good idea to describe the colors you can see when looking at the sandpit.
When describing the colors, you could even describe the color of the sand that the sandpit is filled with, as it could be dark sand, red sand, or bright yellow sand. If you choose to describe the color of the sand, you could also expand a bit further on the theme of sand and describe its consistency. Is it fine sand, or is it coarse sand? Does it feel soft to touch? Is the sandpit filled with sand all the way to the top? Has it perhaps even spilled over a little? Or could the sandpit do with a bit more sand? You could also describe any other items you can see in the sandpit, such as buckets, shovels, and other kids' toys.