Things are more complicated than they look here . First of all, the field tree does not show the relational structure of the data, but rather has a node for each table that is added in the data structure. We just recently had a post on StackOverflow tackling that topic.
Now to your structure - the actual table structure looks like this:
As the SubChildren are a List<LLFormChild2> they need to be added as actual table to the structure - and so do all its parents (SingleChild in this case). As soon as you remove the SubChildren from the structure you’ll get the flat view you’re after. But if the view is not actually flat, the field tree behaves as designed and shows a node for each table in the data structure.
I figure this sounds quite complicated (and is, indeed, your structure is non trivial at second sight) - feel free to follow up with any questions you might have.