Stitching and dataloaders

[SGStino]

Consider a parent-children relation between entities, but the parents reside in one service, the children in another.

Only the children know who their parent is, how can we create a stitched query where we fetch all children for all parents found in a filtered/paged parent query?
the children API has a method getChildrenByParentIds([parentID])

Without stitching this would be done with a dataloader i'd imagine, but with stitching, i fail to see a way to acchieve this without resorting to 1 child query getChildByParentId(parentID) per parent found?

I've seen in the documentation that v12 now uses a query plan? But I fail to see how we can convince that query plan to do batched/grouped lookups yet?