As I'm working on rivers and their basins I thought I might try to just add child relations to a parent relation - but it did not render!
Take Lena river,
1. make a relation for the whole length of the river
2. find some tributaries (Vitim, Kirenga, Olyokma) and repeat step 1
3. make a river basin relation and add relations Lena, Vitim, Kirenga, Olyokma
4. test it
It did not render. It only renders when adding all single ways of all rivers (1).