select row_to_json(t)
from (
select "ParentTable"."parentTableFirstProp", (
select array_to_json(array_agg(row_to_json(child)))
from (
select "childTableFirstProp"
from "ChildTable"
where "ChildTable"."id"="ParentTable"."parentToChildReference"
) child
) as parentToChildReference
from "ParentTable"
) t