There is an event called '"on_sort_children"' for each container object. This event is supposed to be used to get notified, when resorting is needed (in the sourcecode of godot this is used whenever the container itself is resized for example -> than the children need to get repositioned as well)
The event system in godot works by first calling the builtin functionality though. This means when the children need to be resorted, (new children added and the container needs to draw. NOT IMMEDIATLY AFTER ADDING EACH CHILD. FOR BETTER PERFORMANCE. BECAUSE 10 CHILDS CAN BE ADDED AND IT SORTS ONLY ONCE) the builtin funciton gets called first. This function actually transforms the children to the intended locations and sizes. After that the gd script signal connected functions get called. Right after the position actually got updated! Exacly where you need it :)