I don't know Affinity's terminology. But say you have two curves. Drag one on the layers panel until it's a child of the other curve. This can be 'released' by right-clicking the child curve in the Layers Panel. (In other software I'd call this grouping a compound curve, but it doesn't seem to be the case here!)
It doesn't seem to be possible to just click on the parent and 'release child objects' -- this would be *very* useful when breaking up imports from, say, Excel charts where you might be trying to release hundreds of items at a time. Equally there's no way to assign a keyboard shortcut to release these child curves -- 'Release' only applies to Designer's idea of compound curves as made in the 'Layers' menu.
The intuitive way would seem to be by clicking on the parent item and release everything below that level.