Q.What will grow in sand in the shade?
I have just moved house (in a rental, so I can’t make a lot of changes) and there is a sandpit on the back patio (“garden bed full of sand” was how it was worded by the estate agent). Now I can mix in some potting mix so that is not 100% sand (as it isn’t full), but I can’t move it from its full-shade position. Is there anything I can grow in it? I am in a temperate zone

There are a few plants that can grow in sand; like succulents, cactus, and a few perennials: there are vegetables that could also grow in sandy soil, but the full shade is not going to allow much success. The would need at least 6 hours of full sun. Some can take a bit less sunlight.
This actually looks like a child's sandbox.
I'm listing a few articles that will give you more information.
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/amending-sandy-soil.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/selecting-plants-for-sandy-soil.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/selecting-plants-for-sandy-soil.htm