Set a taxonomy to a menu referencing other Taxonomy field reference

I am doing a serious project and I do need to get some help here. I have a taxonomy, let's call it PARENT. Well, in another taxonomy list (CHILD) i have a custom field that references the PARENT taxonomy (e.g. child1 -- parent1 ; chil2 -- parent1 ; child3 --parent1 ; child4 -- parent2 ; child5 -- parent2). Well, how can i made this automatically? Because when I edit a CHILD taxonomy it sends it to the bottom of the menu, here you have an example

Before editing child2 taxonomy
MENU
- PARENT1
-- CHILD1
-- CHILD2
-- CHILD3
- PARENT2
-- CHILD4
-- CHILD5

After editing child2 taxonomy
MENU
- PARENT1
-- CHILD1
-- CHILD3
- PARENT2
-- CHILD4
-- CHILD5
- CHILD2

I need this because I will not be editing those CHILDS and the user will not have access to the menu to change the CHILD2 inside the PARENT1.
EDIT:
PARENT and CHILD are totally different taxonomy lists. I have one field in the CHILD list that references the PARENT one.