How can I learn to sit cross-legged again? [Any exercise physiologists?]

I’m talking about how school children sit on the floor and listen to stories, or kind of how people doing yoga sit but without the weird “both feet on top” thing.

I used to sit like this all the time but lately it seems like my legs aren’t flexible enough and stick up in the air in a “V” shape.

Does anyone know what sort of stretching I can do? I’m only 20, by the way.